1. 25 9月, 2015 4 次提交
    • D
      KVM: disable halt_poll_ns as default for s390x · 920552b2
      David Hildenbrand 提交于
      We observed some performance degradation on s390x with dynamic
      halt polling. Until we can provide a proper fix, let's enable
      halt_poll_ns as default only for supported architectures.
      
      Architectures are now free to set their own halt_poll_ns
      default value.
      Signed-off-by: NDavid Hildenbrand <dahi@linux.vnet.ibm.com>
      Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
      920552b2
    • P
      KVM: x86: fix off-by-one in reserved bits check · 58c95070
      Paolo Bonzini 提交于
      29ecd660 ("KVM: x86: avoid uninitialized variable warning",
      2015-09-06) introduced a not-so-subtle problem, which probably
      escaped review because it was not part of the patch context.
      
      Before the patch, leaf was always equal to iterator.level.  After,
      it is equal to iterator.level - 1 in the call to is_shadow_zero_bits_set,
      and when is_shadow_zero_bits_set does another "-1" the check on
      reserved bits becomes incorrect.  Using "iterator.level" in the call
      fixes this call trace:
      
      WARNING: CPU: 2 PID: 17000 at arch/x86/kvm/mmu.c:3385 handle_mmio_page_fault.part.93+0x1a/0x20 [kvm]()
      Modules linked in: tun sha256_ssse3 sha256_generic drbg binfmt_misc ipv6 vfat fat fuse dm_crypt dm_mod kvm_amd kvm crc32_pclmul aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd fam15h_power amd64_edac_mod k10temp edac_core amdkfd amd_iommu_v2 radeon acpi_cpufreq
      [...]
      Call Trace:
        dump_stack+0x4e/0x84
        warn_slowpath_common+0x95/0xe0
        warn_slowpath_null+0x1a/0x20
        handle_mmio_page_fault.part.93+0x1a/0x20 [kvm]
        tdp_page_fault+0x231/0x290 [kvm]
        ? emulator_pio_in_out+0x6e/0xf0 [kvm]
        kvm_mmu_page_fault+0x36/0x240 [kvm]
        ? svm_set_cr0+0x95/0xc0 [kvm_amd]
        pf_interception+0xde/0x1d0 [kvm_amd]
        handle_exit+0x181/0xa70 [kvm_amd]
        ? kvm_arch_vcpu_ioctl_run+0x68b/0x1730 [kvm]
        kvm_arch_vcpu_ioctl_run+0x6f6/0x1730 [kvm]
        ? kvm_arch_vcpu_ioctl_run+0x68b/0x1730 [kvm]
        ? preempt_count_sub+0x9b/0xf0
        ? mutex_lock_killable_nested+0x26f/0x490
        ? preempt_count_sub+0x9b/0xf0
        kvm_vcpu_ioctl+0x358/0x710 [kvm]
        ? __fget+0x5/0x210
        ? __fget+0x101/0x210
        do_vfs_ioctl+0x2f4/0x560
        ? __fget_light+0x29/0x90
        SyS_ioctl+0x4c/0x90
        entry_SYSCALL_64_fastpath+0x16/0x73
      ---[ end trace 37901c8686d84de6 ]---
      Reported-by: NBorislav Petkov <bp@alien8.de>
      Tested-by: NBorislav Petkov <bp@alien8.de>
      Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
      58c95070
    • P
      KVM: x86: use correct page table format to check nested page table reserved bits · 6fec2144
      Paolo Bonzini 提交于
      Intel CPUID on AMD host or vice versa is a weird case, but it can
      happen.  Handle it by checking the host CPU vendor instead of the
      guest's in reset_tdp_shadow_zero_bits_mask.  For speed, the
      check uses the fact that Intel EPT has an X (executable) bit while
      AMD NPT has NX.
      Reported-by: NBorislav Petkov <bp@alien8.de>
      Tested-by: NBorislav Petkov <bp@alien8.de>
      Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
      6fec2144
    • P
      KVM: svm: do not call kvm_set_cr0 from init_vmcb · 79a8059d
      Paolo Bonzini 提交于
      kvm_set_cr0 may want to call kvm_zap_gfn_range and thus access the
      memslots array (SRCU protected).  Using a mini SRCU critical section
      is ugly, and adding it to kvm_arch_vcpu_create doesn't work because
      the VMX vcpu_create callback calls synchronize_srcu.
      
      Fixes this lockdep splat:
      
      ===============================
      [ INFO: suspicious RCU usage. ]
      4.3.0-rc1+ #1 Not tainted
      -------------------------------
      include/linux/kvm_host.h:488 suspicious rcu_dereference_check() usage!
      
      other info that might help us debug this:
      rcu_scheduler_active = 1, debug_locks = 0
      1 lock held by qemu-system-i38/17000:
       #0:  (&(&kvm->mmu_lock)->rlock){+.+...}, at: kvm_zap_gfn_range+0x24/0x1a0 [kvm]
      
      [...]
      Call Trace:
       dump_stack+0x4e/0x84
       lockdep_rcu_suspicious+0xfd/0x130
       kvm_zap_gfn_range+0x188/0x1a0 [kvm]
       kvm_set_cr0+0xde/0x1e0 [kvm]
       init_vmcb+0x760/0xad0 [kvm_amd]
       svm_create_vcpu+0x197/0x250 [kvm_amd]
       kvm_arch_vcpu_create+0x47/0x70 [kvm]
       kvm_vm_ioctl+0x302/0x7e0 [kvm]
       ? __lock_is_held+0x51/0x70
       ? __fget+0x101/0x210
       do_vfs_ioctl+0x2f4/0x560
       ? __fget_light+0x29/0x90
       SyS_ioctl+0x4c/0x90
       entry_SYSCALL_64_fastpath+0x16/0x73
      Reported-by: NBorislav Petkov <bp@alien8.de>
      Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
      79a8059d
  2. 23 9月, 2015 14 次提交
  3. 21 9月, 2015 1 次提交
  4. 18 9月, 2015 1 次提交
    • I
      kvm: svm: reset mmu on VCPU reset · ebae871a
      Igor Mammedov 提交于
      When INIT/SIPI sequence is sent to VCPU which before that
      was in use by OS, VMRUN might fail with:
      
       KVM: entry failed, hardware error 0xffffffff
       EAX=00000000 EBX=00000000 ECX=00000000 EDX=000006d3
       ESI=00000000 EDI=00000000 EBP=00000000 ESP=00000000
       EIP=00000000 EFL=00000002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
       ES =0000 00000000 0000ffff 00009300
       CS =9a00 0009a000 0000ffff 00009a00
       [...]
       CR0=60000010 CR2=b6f3e000 CR3=01942000 CR4=000007e0
       [...]
       EFER=0000000000000000
      
      with corresponding SVM error:
       KVM: FAILED VMRUN WITH VMCB:
       [...]
       cpl:            0                efer:         0000000000001000
       cr0:            0000000080010010 cr2:          00007fd7fe85bf90
       cr3:            0000000187d0c000 cr4:          0000000000000020
       [...]
      
      What happens is that VCPU state right after offlinig:
      CR0: 0x80050033  EFER: 0xd01  CR4: 0x7e0
        -> long mode with CR3 pointing to longmode page tables
      
      and when VCPU gets INIT/SIPI following transition happens
      CR0: 0 -> 0x60000010 EFER: 0x0  CR4: 0x7e0
        -> paging disabled with stale CR3
      
      However SVM under the hood puts VCPU in Paged Real Mode*
      which effectively translates CR0 0x60000010 -> 80010010 after
      
         svm_vcpu_reset()
             -> init_vmcb()
                 -> kvm_set_cr0()
                     -> svm_set_cr0()
      
      but from  kvm_set_cr0() perspective CR0: 0 -> 0x60000010
      only caching bits are changed and
      commit d81135a5
       ("KVM: x86: do not reset mmu if CR0.CD and CR0.NW are changed")'
      regressed svm_vcpu_reset() which relied on MMU being reset.
      
      As result VMRUN after svm_vcpu_reset() tries to run
      VCPU in Paged Real Mode with stale MMU context (longmode page tables),
      which causes some AMD CPUs** to bail out with VMEXIT_INVALID.
      
      Fix issue by unconditionally resetting MMU context
      at init_vmcb() time.
      
      	* AMD64 Architecture Programmer’s Manual,
      	    Volume 2: System Programming, rev: 3.25
      	      15.19 Paged Real Mode
      	** Opteron 1216
      Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
      Fixes: d81135a5
      Cc: stable@vger.kernel.org
      Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
      ebae871a
  5. 17 9月, 2015 1 次提交
  6. 16 9月, 2015 6 次提交
  7. 15 9月, 2015 2 次提交
    • S
      x86/apic: Serialize LVTT and TSC_DEADLINE writes · 5d7c631d
      Shaohua Li 提交于
      The APIC LVTT register is MMIO mapped but the TSC_DEADLINE register is an
      MSR. The write to the TSC_DEADLINE MSR is not serializing, so it's not
      guaranteed that the write to LVTT has reached the APIC before the
      TSC_DEADLINE MSR is written. In such a case the write to the MSR is
      ignored and as a consequence the local timer interrupt never fires.
      
      The SDM decribes this issue for xAPIC and x2APIC modes. The
      serialization methods recommended by the SDM differ.
      
      xAPIC:
       "1. Memory-mapped write to LVT Timer Register, setting bits 18:17 to 10b.
        2. WRMSR to the IA32_TSC_DEADLINE MSR a value much larger than current time-stamp counter.
        3. If RDMSR of the IA32_TSC_DEADLINE MSR returns zero, go to step 2.
        4. WRMSR to the IA32_TSC_DEADLINE MSR the desired deadline."
      
      x2APIC:
       "To allow for efficient access to the APIC registers in x2APIC mode,
        the serializing semantics of WRMSR are relaxed when writing to the
        APIC registers. Thus, system software should not use 'WRMSR to APIC
        registers in x2APIC mode' as a serializing instruction. Read and write
        accesses to the APIC registers will occur in program order. A WRMSR to
        an APIC register may complete before all preceding stores are globally
        visible; software can prevent this by inserting a serializing
        instruction, an SFENCE, or an MFENCE before the WRMSR."
      
      The xAPIC method is to just wait for the memory mapped write to hit
      the LVTT by checking whether the MSR write has reached the hardware.
      There is no reason why a proper MFENCE after the memory mapped write would
      not do the same. Andi Kleen confirmed that MFENCE is sufficient for the
      xAPIC case as well.
      
      Issue MFENCE before writing to the TSC_DEADLINE MSR. This can be done
      unconditionally as all CPUs which have TSC_DEADLINE also have MFENCE
      support.
      
      [ tglx: Massaged the changelog ]
      Signed-off-by: NShaohua Li <shli@fb.com>
      Reviewed-by: NIngo Molnar <mingo@kernel.org>
      Cc: <Kernel-team@fb.com>
      Cc: <lenb@kernel.org>
      Cc: <fenghua.yu@intel.com>
      Cc: Andi Kleen <ak@linux.intel.com>
      Cc: H. Peter Anvin <hpa@zytor.com>
      Cc: stable@vger.kernel.org #v3.7+
      Link: http://lkml.kernel.org/r/20150909041352.GA2059853@devbig257.prn2.facebook.comSigned-off-by: NThomas Gleixner <tglx@linutronix.de>
      5d7c631d
    • T
      x86/ioapic: Force affinity setting in setup_ioapic_dest() · 4857c91f
      Thomas Gleixner 提交于
      The recent ioapic cleanups changed the affinity setting in
      setup_ioapic_dest() from a direct write to the hardware to the delayed
      affinity setup via irq_set_affinity().
      
      That results in a warning from chained_irq_exit():
      WARNING: CPU: 0 PID: 5 at kernel/irq/migration.c:32 irq_move_masked_irq
      [<ffffffff810a0a88>] irq_move_masked_irq+0xb8/0xc0
      [<ffffffff8103c161>] ioapic_ack_level+0x111/0x130
      [<ffffffff812bbfe8>] intel_gpio_irq_handler+0x148/0x1c0
      
      The reason is that irq_set_affinity() does not write directly to the
      hardware. It marks the affinity setting as pending and executes it
      from the next interrupt. The chained handler infrastructure does not
      take the irq descriptor lock for performance reasons because such a
      chained interrupt is not visible to any interfaces. So the delayed
      affinity setting triggers the warning in irq_move_masked_irq().
      
      Restore the old behaviour by calling the set_affinity function of the
      ioapic chip in setup_ioapic_dest(). This is safe as none of the
      interrupts can be on the fly at this point.
      
      Fixes: aa5cb97f 'x86/irq: Remove x86_io_apic_ops.set_affinity and related interfaces'
      Reported-and-tested-by: NMika Westerberg <mika.westerberg@linux.intel.com>
      Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
      Cc: Jiang Liu <jiang.liu@linux.intel.com>
      Cc: jarkko.nikula@linux.intel.com
      4857c91f
  8. 14 9月, 2015 3 次提交
    • J
      x86/paravirt: Remove the unused pv_time_ops::get_tsc_khz method · cda34fc7
      Juergen Gross 提交于
      It's not used anywhere.
      Signed-off-by: NJuergen Gross <jgross@suse.com>
      Acked-by: NRusty Russell <rusty@rustcorp.com.au>
      Cc: Andy Lutomirski <luto@amacapital.net>
      Cc: Borislav Petkov <bp@alien8.de>
      Cc: Brian Gerst <brgerst@gmail.com>
      Cc: Denys Vlasenko <dvlasenk@redhat.com>
      Cc: H. Peter Anvin <hpa@zytor.com>
      Cc: Linus Torvalds <torvalds@linux-foundation.org>
      Cc: Peter Zijlstra <peterz@infradead.org>
      Cc: Thomas Gleixner <tglx@linutronix.de>
      Cc: akataria@vmware.com
      Cc: chrisw@sous-sol.org
      Cc: jeremy@goop.org
      Cc: virtualization@lists.linux-foundation.org
      Link: http://lkml.kernel.org/r/1442227343-403-1-git-send-email-jgross@suse.comSigned-off-by: NIngo Molnar <mingo@kernel.org>
      cda34fc7
    • J
      x86/ldt: Fix small LDT allocation for Xen · f454b478
      Jan Beulich 提交于
      While the following commit:
      
        37868fe1 ("x86/ldt: Make modify_ldt synchronous")
      
      added a nice comment explaining that Xen needs page-aligned
      whole page chunks for guest descriptor tables, it then
      nevertheless used kzalloc() on the small size path.
      
      As I'm unaware of guarantees for kmalloc(PAGE_SIZE, ) to return
      page-aligned memory blocks, I believe this needs to be switched
      back to __get_free_page() (or better get_zeroed_page()).
      Signed-off-by: NJan Beulich <jbeulich@suse.com>
      Cc: Andy Lutomirski <luto@amacapital.net>
      Cc: Andy Lutomirski <luto@kernel.org>
      Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
      Cc: Borislav Petkov <bp@alien8.de>
      Cc: Brian Gerst <brgerst@gmail.com>
      Cc: David Vrabel <david.vrabel@citrix.com>
      Cc: Denys Vlasenko <dvlasenk@redhat.com>
      Cc: H. Peter Anvin <hpa@zytor.com>
      Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
      Cc: Linus Torvalds <torvalds@linux-foundation.org>
      Cc: Peter Zijlstra <peterz@infradead.org>
      Cc: Thomas Gleixner <tglx@linutronix.de>
      Link: http://lkml.kernel.org/r/55E735D6020000780009F1E6@prv-mh.provo.novell.comSigned-off-by: NIngo Molnar <mingo@kernel.org>
      f454b478
    • I
      x86/vm86: Fix the misleading CONFIG_VM86 Kconfig help text · 1e642812
      Ingo Molnar 提交于
      The CONFIG_VM86 Kconfig help text is actively misleading, so fix it:
      
        - Don't mark it 'obsolete' in the text as we'll support the ABI as long as CPUs
          support it.
      
        - Qualify the part about software emulation and mention that for some apps you
          want a real vm86 mode.
      
        - Don't scare users away from the option, instead explain what it does.
      Reported-by: NStas Sergeev <stsp@list.ru>
      Cc: Andy Lutomirski <luto@kernel.org>
      Cc: Arjan van de Ven <arjan@linux.intel.com>
      Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>
      Cc: Borislav Petkov <bp@alien8.de>
      Cc: Brian Gerst <brgerst@gmail.com>
      Cc: Josh Boyer <jwboyer@fedoraproject.org>
      Cc: Kees Cook <keescook@chromium.org>
      Cc: Linus Torvalds <torvalds@linux-foundation.org>
      Cc: Matthew Garrett <mjg59@srcf.ucam.org>
      Cc: Oleg Nesterov <oleg@redhat.com>
      Cc: Peter Zijlstra <peterz@infradead.org>
      Cc: Thomas Gleixner <tglx@linutronix.de>
      Cc: linux-kernel@vger.kernel.org
      Signed-off-by: NIngo Molnar <mingo@kernel.org>
      1e642812
  9. 13 9月, 2015 2 次提交
  10. 12 9月, 2015 1 次提交
    • M
      sys_membarrier(): system-wide memory barrier (generic, x86) · 5b25b13a
      Mathieu Desnoyers 提交于
      Here is an implementation of a new system call, sys_membarrier(), which
      executes a memory barrier on all threads running on the system.  It is
      implemented by calling synchronize_sched().  It can be used to
      distribute the cost of user-space memory barriers asymmetrically by
      transforming pairs of memory barriers into pairs consisting of
      sys_membarrier() and a compiler barrier.  For synchronization primitives
      that distinguish between read-side and write-side (e.g.  userspace RCU
      [1], rwlocks), the read-side can be accelerated significantly by moving
      the bulk of the memory barrier overhead to the write-side.
      
      The existing applications of which I am aware that would be improved by
      this system call are as follows:
      
      * Through Userspace RCU library (http://urcu.so)
        - DNS server (Knot DNS) https://www.knot-dns.cz/
        - Network sniffer (http://netsniff-ng.org/)
        - Distributed object storage (https://sheepdog.github.io/sheepdog/)
        - User-space tracing (http://lttng.org)
        - Network storage system (https://www.gluster.org/)
        - Virtual routers (https://events.linuxfoundation.org/sites/events/files/slides/DPDK_RCU_0MQ.pdf)
        - Financial software (https://lkml.org/lkml/2015/3/23/189)
      
      Those projects use RCU in userspace to increase read-side speed and
      scalability compared to locking.  Especially in the case of RCU used by
      libraries, sys_membarrier can speed up the read-side by moving the bulk of
      the memory barrier cost to synchronize_rcu().
      
      * Direct users of sys_membarrier
        - core dotnet garbage collector (https://github.com/dotnet/coreclr/issues/198)
      
      Microsoft core dotnet GC developers are planning to use the mprotect()
      side-effect of issuing memory barriers through IPIs as a way to implement
      Windows FlushProcessWriteBuffers() on Linux.  They are referring to
      sys_membarrier in their github thread, specifically stating that
      sys_membarrier() is what they are looking for.
      
      To explain the benefit of this scheme, let's introduce two example threads:
      
      Thread A (non-frequent, e.g. executing liburcu synchronize_rcu())
      Thread B (frequent, e.g. executing liburcu
      rcu_read_lock()/rcu_read_unlock())
      
      In a scheme where all smp_mb() in thread A are ordering memory accesses
      with respect to smp_mb() present in Thread B, we can change each
      smp_mb() within Thread A into calls to sys_membarrier() and each
      smp_mb() within Thread B into compiler barriers "barrier()".
      
      Before the change, we had, for each smp_mb() pairs:
      
      Thread A                    Thread B
      previous mem accesses       previous mem accesses
      smp_mb()                    smp_mb()
      following mem accesses      following mem accesses
      
      After the change, these pairs become:
      
      Thread A                    Thread B
      prev mem accesses           prev mem accesses
      sys_membarrier()            barrier()
      follow mem accesses         follow mem accesses
      
      As we can see, there are two possible scenarios: either Thread B memory
      accesses do not happen concurrently with Thread A accesses (1), or they
      do (2).
      
      1) Non-concurrent Thread A vs Thread B accesses:
      
      Thread A                    Thread B
      prev mem accesses
      sys_membarrier()
      follow mem accesses
                                  prev mem accesses
                                  barrier()
                                  follow mem accesses
      
      In this case, thread B accesses will be weakly ordered. This is OK,
      because at that point, thread A is not particularly interested in
      ordering them with respect to its own accesses.
      
      2) Concurrent Thread A vs Thread B accesses
      
      Thread A                    Thread B
      prev mem accesses           prev mem accesses
      sys_membarrier()            barrier()
      follow mem accesses         follow mem accesses
      
      In this case, thread B accesses, which are ensured to be in program
      order thanks to the compiler barrier, will be "upgraded" to full
      smp_mb() by synchronize_sched().
      
      * Benchmarks
      
      On Intel Xeon E5405 (8 cores)
      (one thread is calling sys_membarrier, the other 7 threads are busy
      looping)
      
      1000 non-expedited sys_membarrier calls in 33s =3D 33 milliseconds/call.
      
      * User-space user of this system call: Userspace RCU library
      
      Both the signal-based and the sys_membarrier userspace RCU schemes
      permit us to remove the memory barrier from the userspace RCU
      rcu_read_lock() and rcu_read_unlock() primitives, thus significantly
      accelerating them. These memory barriers are replaced by compiler
      barriers on the read-side, and all matching memory barriers on the
      write-side are turned into an invocation of a memory barrier on all
      active threads in the process. By letting the kernel perform this
      synchronization rather than dumbly sending a signal to every process
      threads (as we currently do), we diminish the number of unnecessary wake
      ups and only issue the memory barriers on active threads. Non-running
      threads do not need to execute such barrier anyway, because these are
      implied by the scheduler context switches.
      
      Results in liburcu:
      
      Operations in 10s, 6 readers, 2 writers:
      
      memory barriers in reader:    1701557485 reads, 2202847 writes
      signal-based scheme:          9830061167 reads,    6700 writes
      sys_membarrier:               9952759104 reads,     425 writes
      sys_membarrier (dyn. check):  7970328887 reads,     425 writes
      
      The dynamic sys_membarrier availability check adds some overhead to
      the read-side compared to the signal-based scheme, but besides that,
      sys_membarrier slightly outperforms the signal-based scheme. However,
      this non-expedited sys_membarrier implementation has a much slower grace
      period than signal and memory barrier schemes.
      
      Besides diminishing the number of wake-ups, one major advantage of the
      membarrier system call over the signal-based scheme is that it does not
      need to reserve a signal. This plays much more nicely with libraries,
      and with processes injected into for tracing purposes, for which we
      cannot expect that signals will be unused by the application.
      
      An expedited version of this system call can be added later on to speed
      up the grace period. Its implementation will likely depend on reading
      the cpu_curr()->mm without holding each CPU's rq lock.
      
      This patch adds the system call to x86 and to asm-generic.
      
      [1] http://urcu.so
      
      membarrier(2) man page:
      
      MEMBARRIER(2)              Linux Programmer's Manual             MEMBARRIER(2)
      
      NAME
             membarrier - issue memory barriers on a set of threads
      
      SYNOPSIS
             #include <linux/membarrier.h>
      
             int membarrier(int cmd, int flags);
      
      DESCRIPTION
             The cmd argument is one of the following:
      
             MEMBARRIER_CMD_QUERY
                    Query  the  set  of  supported commands. It returns a bitmask of
                    supported commands.
      
             MEMBARRIER_CMD_SHARED
                    Execute a memory barrier on all threads running on  the  system.
                    Upon  return from system call, the caller thread is ensured that
                    all running threads have passed through a state where all memory
                    accesses  to  user-space  addresses  match program order between
                    entry to and return from the system  call  (non-running  threads
                    are de facto in such a state). This covers threads from all pro=E2=80=90
                    cesses running on the system.  This command returns 0.
      
             The flags argument needs to be 0. For future extensions.
      
             All memory accesses performed  in  program  order  from  each  targeted
             thread is guaranteed to be ordered with respect to sys_membarrier(). If
             we use the semantic "barrier()" to represent a compiler barrier forcing
             memory  accesses  to  be performed in program order across the barrier,
             and smp_mb() to represent explicit memory barriers forcing full  memory
             ordering  across  the barrier, we have the following ordering table for
             each pair of barrier(), sys_membarrier() and smp_mb():
      
             The pair ordering is detailed as (O: ordered, X: not ordered):
      
                                    barrier()   smp_mb() sys_membarrier()
                    barrier()          X           X            O
                    smp_mb()           X           O            O
                    sys_membarrier()   O           O            O
      
      RETURN VALUE
             On success, these system calls return zero.  On error, -1 is  returned,
             and errno is set appropriately. For a given command, with flags
             argument set to 0, this system call is guaranteed to always return the
             same value until reboot.
      
      ERRORS
             ENOSYS System call is not implemented.
      
             EINVAL Invalid arguments.
      
      Linux                             2015-04-15                     MEMBARRIER(2)
      Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@efficios.com>
      Reviewed-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
      Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
      Cc: Steven Rostedt <rostedt@goodmis.org>
      Cc: Nicholas Miell <nmiell@comcast.net>
      Cc: Ingo Molnar <mingo@redhat.com>
      Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
      Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
      Cc: Stephen Hemminger <stephen@networkplumber.org>
      Cc: Thomas Gleixner <tglx@linutronix.de>
      Cc: Peter Zijlstra <peterz@infradead.org>
      Cc: David Howells <dhowells@redhat.com>
      Cc: Pranith Kumar <bobby.prani@gmail.com>
      Cc: Michael Kerrisk <mtk.manpages@gmail.com>
      Cc: Shuah Khan <shuahkh@osg.samsung.com>
      Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
      5b25b13a
  11. 11 9月, 2015 5 次提交