When running the tsc_adjust kvm-unit-test on an AMD processor with the
IA32_TSC_ADJUST feature enabled, the WARN_ON in svm_adjust_tsc_offset can be
triggered. This WARN_ON checks for a negative adjustment in case __scale_tsc
is called; however it may trigger unnecessary warnings.

This patch moves the WARN_ON to trigger only if __scale_tsc will actually be
called from svm_adjust_tsc_offset. In addition make adj in kvm_set_msr_common
s64 since this can have signed values.
Signed-off-by: NChris J Arges <chris.j.arges@canonical.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

There's nothing to switch if the host and guest values are the same.
I am unable to find evidence that this makes any difference
whatsoever.
Signed-off-by: NAndy Lutomirski <luto@amacapital.net>
[I could see a difference on Nehalem.  From 5 runs:

 userspace exit, guest!=host   12200 11772 12130 12164 12327
 userspace exit, guest=host    11983 11780 11920 11919 12040
 lightweight exit, guest!=host  3214  3220  3238  3218  3337
 lightweight exit, guest=host   3178  3193  3193  3187  3220

 This passes the t-test with 99% confidence for userspace exit,
 98.5% confidence for lightweight exit. - Paolo]
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

At least on Sandy Bridge, letting the CPU switch IA32_EFER is much
faster than switching it manually.

I benchmarked this using the vmexit kvm-unit-test (single run, but
GOAL multiplied by 5 to do more iterations):

Test                                  Before      After    Change
cpuid                                   2000       1932    -3.40%
vmcall                                  1914       1817    -5.07%
mov_from_cr8                              13         13     0.00%
mov_to_cr8                                19         19     0.00%
inl_from_pmtimer                       19164      10619   -44.59%
inl_from_qemu                          15662      10302   -34.22%
inl_from_kernel                         3916       3802    -2.91%
outl_to_kernel                          2230       2194    -1.61%
mov_dr                                   172        176     2.33%
ipi                                (skipped)  (skipped)
ipi+halt                           (skipped)  (skipped)
ple-round-robin                           13         13     0.00%
wr_tsc_adjust_msr                       1920       1845    -3.91%
rd_tsc_adjust_msr                       1892       1814    -4.12%
mmio-no-eventfd:pci-mem                16394      11165   -31.90%
mmio-wildcard-eventfd:pci-mem           4607       4645     0.82%
mmio-datamatch-eventfd:pci-mem          4601       4610     0.20%
portio-no-eventfd:pci-io               11507       7942   -30.98%
portio-wildcard-eventfd:pci-io          2239       2225    -0.63%
portio-datamatch-eventfd:pci-io         2250       2234    -0.71%

I haven't explicitly computed the significance of these numbers,
but this isn't subtle.
Signed-off-by: NAndy Lutomirski <luto@amacapital.net>
[The results were reproducible on all of Nehalem, Sandy Bridge and
 Ivy Bridge.  The slowness of manual switching is because writing
 to EFER with WRMSR triggers a TLB flush, even if the only bit you're
 touching is SCE (so the page table format is not affected).  Doing
 the write as part of vmentry/vmexit, instead, does not flush the TLB,
 probably because all processors that have EPT also have VPID. - Paolo]
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

PCIDs are only supported in 64-bit mode.  No need to clear bit 63
of CR3 unless the host is 64-bit.

Reported by Fengguang Wu's autobuilder.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

The new trace event records:
  * the id of vcpu being updated
  * the pvclock_vcpu_time_info struct being written to guest memory

This is useful for debugging pvclock bugs, such as the bug fixed by
"[PATCH] kvm: x86: Fix kvm clock versioning.".
Signed-off-by: NDavid Matlack <dmatlack@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

kvm updates the version number for the guest paravirt clock structure by
incrementing the version of its private copy. It does not read the guest
version, so will write version = 2 in the first update for every new VM,
including after restoring a saved state. If guest state is saved during
reading the clock, it could read and accept struct fields and guest TSC
from two different updates. This changes the code to increment the guest
version and write it back.
Signed-off-by: NOwen Hofmann <osh@google.com>
Reviewed-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Commit 3b32004a ("KVM: x86: movnti minimum op size of 32-bit is not kept")
did not fully fix the minimum operand size of MONTI emulation. Still, MOVNTI
may be mistakenly performed using 16-bit opsize.

This patch add No16 flag to mark an instruction does not support 16-bits
operand size.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

When the guest writes to the TSC, the masterclock TSC copy must be
updated as well along with the TSC_OFFSET update, otherwise a negative
tsc_timestamp is calculated at kvm_guest_time_update.

Once "if (!vcpus_matched && ka->use_master_clock)" is simplified to
"if (ka->use_master_clock)", the corresponding "if (!ka->use_master_clock)"
becomes redundant, so remove the do_request boolean and collapse
everything into a single condition.
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Now that KVM injects #UD on "unhandlable" error, it makes better sense to
return such error on sysenter instead of directly injecting #UD to the guest.
This allows to track more easily the unhandlable cases the emulator does not
support.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

APIC base relocation is unsupported by KVM. If anyone uses it, the least should
be to report a warning in the hypervisor.

Note that KVM-unit-tests uses this feature for some reason, so running the
tests triggers the warning.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Commit 7fe864dc (KVM: x86: Mark VEX-prefix instructions emulation as
unimplemented, 2014-06-02) marked VEX instructions as such in protected
mode.  VEX-prefix instructions are not supported relevant on real-mode
and VM86, but should cause #UD instead of being decoded as LES/LDS.

Fix this behaviour to be consistent with real hardware.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
[Check for mod == 3, rather than 2 or 3. - Paolo]
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Task-switch emulation checks the privilege level prior to performing the
task-switch.  This check is incorrect in the case of task-gates, in which the
tss.dpl is ignored, and can cause superfluous exceptions.  Moreover this check
is unnecassary, since the CPU checks the privilege levels prior to exiting.
Intel SDM 25.4.2 says "If CALL or JMP accesses a TSS descriptor directly
outside IA-32e mode, privilege levels are checked on the TSS descriptor" prior
to exiting.  AMD 15.14.1 says "The intercept is checked before the task switch
takes place but after the incoming TSS and task gate (if one was involved) have
been checked for correctness."

This patch removes the CPL checks for CALL and JMP.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

When emulating LTR/LDTR/LGDT/LIDT, #GP should be injected if the base is
non-canonical. Otherwise, VM-entry will fail.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

LGDT and LIDT emulation logic is almost identical. Merge the logic into a
single point to avoid redundancy. This will be used by the next patch that
will ensure the bases of the loaded GDTR and IDTR are canonical.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

If the emulation ends in fault, eflags should not be updated.  However, several
instruction emulations (actually all the fastops) currently update eflags, if
the fault was detected afterwards (e.g., #PF during writeback).
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Although Intel SDM mentions bit 63 is reserved, MOV to CR3 can have bit 63 set.
As Intel SDM states in section 4.10.4 "Invalidation of TLBs and
Paging-Structure Caches": " MOV to CR3. ... If CR4.PCIDE = 1 and bit 63 of the
instruction’s source operand is 0 ..."

In other words, bit 63 is not reserved. KVM emulator currently consider bit 63
as reserved. Fix it.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

According to Intel SDM push of segment selectors is done in the following
manner: "if the operand size is 32-bits, either a zero-extended value is pushed
on the stack or the segment selector is written on the stack using a 16-bit
move. For the last case, all recent Core and Atom processors perform a 16-bit
move, leaving the upper portion of the stack location unmodified."

This patch modifies the behavior to match the core behavior.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

CMPS and SCAS instructions are evaluated in the wrong order.  For reference (of
CMPS), see http://www.fermimn.gov.it/linux/quarta/x86/cmps.htm : "Note that the
direction of subtraction for CMPS is [SI] - [DI] or [ESI] - [EDI]. The left
operand (SI or ESI) is the source and the right operand (DI or EDI) is the
destination. This is the reverse of the usual Intel convention in which the
left operand is the destination and the right operand is the source."

Introducing em_cmp_r for this matter that performs comparison in reverse order
using fastop infrastructure to avoid a wrapper function.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

SYSCALL emulation currently clears in 64-bit mode eflags according to
MSR_SYSCALL_MASK.  However, on bare-metal eflags[1] which is fixed to one
cannot be cleared, even if MSR_SYSCALL_MASK masks the bit.  This wrong behavior
may result in failed VM-entry, as VT disallows entry with eflags[1] cleared.

This patch sets the bit after masking eflags on syscall.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

In x86, you can only MOV-sreg to memory with either 16-bits or 64-bits size.
In contrast, KVM may write to 32-bits memory on MOV-sreg. This patch fixes KVM
behavior, and sets the destination operand size to two, if the destination is
memory.

When destination is registers, and the operand size is 32-bits, the high
16-bits in modern CPUs is filled with zero.  This is handled correctly.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

x86 debug registers hold a linear address. Therefore, breakpoints detection
should consider CS.base, and check whether instruction linear address equals
(CS.base + RIP). This patch introduces a function to evaluate RIP linear
address and uses it for breakpoints detection.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

DR6[0:3] (previous breakpoint indications) are cleared when #DB is injected
during handle_exception, just as real hardware does.  Similarily, handle_dr
should clear DR6[0:3].
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

It should clear B0-B3 and set BD.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Real-mode exceptions do not deliver error code. As can be seen in Intel SDM
volume 2, real-mode exceptions do not have parentheses, which indicate
error-code.  To avoid significant changes of the code, the error code is
"removed" during exception queueing.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

In one occassion, decode_modrm uses the rm field after it is extended with
REX.B to determine the addressing mode. Doing so causes it not to read the
offset for rip-relative addressing with REX.B=1.

This patch moves the fetch where we already mask REX.B away instead.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

A bug was reported as follows: when running Windows 7 32-bit guests on qemu-kvm,
sometimes the guests run into blue screen during reboot. The problem was that a
guest's RVI was not cleared when it rebooted. This patch has fixed the problem.
Signed-off-by: NWei Wang <wei.w.wang@intel.com>
Signed-off-by: NYang Zhang <yang.z.zhang@intel.com>
Tested-by: NRongrong Liu &lt;rongrongx.liu@intel.com&gt;, Da Chun <ngugc@qq.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Return a negative error code instead, and WARN() when we should be covering
the entire 2-bit space of vmcs_field_type's return value.  For increased
robustness, add a BUILD_BUG_ON checking the range of vmcs_field_to_offset.
Suggested-by: NTiejun Chen <tiejun.chen@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Instead of vmx_init(), actually it would make reasonable sense to do
anything specific to vmx hardware setting in vmx_x86_ops->hardware_setup().
Signed-off-by: NTiejun Chen <tiejun.chen@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Just move this pair of functions down to make sure later we can
add something dependent on others.
Signed-off-by: NTiejun Chen <tiejun.chen@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

According to the architecture all instructions are suppressing if memory
access is prohibited due to DAT protection, unless stated otherwise for
an instruction.
The lctl[g]/stctl[g] implementations handled this incorrectly since
control register handling was done piecemeal, which means they had
terminating instead of suppressing semantics.
This patch fixes this.
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Reviewed-by: NThomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: NDavid Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>

some control register changes will flush some aspects of the CPU, e.g.
POP explicitely mentions that for CR9-CR11 "TLBs may be cleared".
Instead of trying to be clever and only flush on specific CRs, let
play safe and flush on all lctl(g) as future machines might define
new bits in CRs. Load control intercept should not happen that often.
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Acked-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: NDavid Hildenbrand <dahi@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org

ipte_unlock_siif uses cmpxchg to replace the in-memory data of the ipte
lock together with ACCESS_ONCE for the intial read.

union ipte_control {
        unsigned long val;
        struct {
                unsigned long k  : 1;
                unsigned long kh : 31;
                unsigned long kg : 32;
        };
};
[...]
static void ipte_unlock_siif(struct kvm_vcpu *vcpu)
{
        union ipte_control old, new, *ic;

        ic = &vcpu->kvm->arch.sca->ipte_control;
        do {
                new = old = ACCESS_ONCE(*ic);
                new.kh--;
                if (!new.kh)
                        new.k = 0;
        } while (cmpxchg(&ic->val, old.val, new.val) != old.val);
        if (!new.kh)
                wake_up(&vcpu->kvm->arch.ipte_wq);
}

The new value, is loaded twice from memory with gcc 4.7.2 of
fedora 18, despite the ACCESS_ONCE:

--->

l       %r4,0(%r3)      <--- load first 32 bit of lock (k and kh) in r4
alfi    %r4,2147483647  <--- add -1 to r4
llgtr   %r4,%r4         <--- zero out the sign bit of r4
lg      %r1,0(%r3)      <--- load all 64 bit of lock into new
lgr     %r2,%r1         <--- load the same into old
risbg   %r1,%r4,1,31,32 <--- shift and insert r4 into the bits 1-31 of
new
llihf   %r4,2147483647
ngrk    %r4,%r1,%r4
jne     aa0 <ipte_unlock+0xf8>
nihh    %r1,32767
lgr     %r4,%r2
csg     %r4,%r1,0(%r3)
cgr     %r2,%r4
jne     a70 <ipte_unlock+0xc8>

If the memory value changes between the first load (l) and the second
load (lg) we are broken. If that happens VCPU threads will hang
(unkillable) in handle_ipte_interlock.

Andreas Krebbel analyzed this and tracked it down to a compiler bug in
that version:
"while it is not that obvious the C99 standard basically forbids
duplicating the memory access also in that case. For an argumentation of
a similiar case please see:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=22278#c43

For the implementation-defined cases regarding volatile there are some
GCC-specific clarifications which can be found here:
https://gcc.gnu.org/onlinedocs/gcc/Volatiles.html#Volatiles

I've tracked down the problem with a reduced testcase. The problem was
that during a tree level optimization (SRA - scalar replacement of
aggregates) the volatile marker is lost. And an RTL level optimizer (CSE
- common subexpression elimination) then propagated the memory read into
  its second use introducing another access to the memory location. So
indeed Christian's suspicion that the union access has something to do
with it is correct (since it triggered the SRA optimization).

This issue has been reported and fixed in the GCC 4.8 development cycle:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145"

This patch replaces the ACCESS_ONCE scheme with a barrier() based scheme
that should work for all supported compilers.
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Cc: stable@vger.kernel.org # v3.16+

We can use get_cpu() and put_cpu() to replace
preempt_disable()/cpu = smp_processor_id() and
preempt_enable() for slightly better code.
Signed-off-by: NTiejun Chen <tiejun.chen@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

We mirror a subset of these registers in separate variables.
Using them directly should be faster.
Signed-off-by: NRadim Krčmář <rkrcmar@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

APIC-write VM exits are "trap-like": they save CS:RIP values for the
instruction after the write, and more importantly, the handler will
already see the new value in the virtual-APIC page.  This means that
apic_reg_write cannot use kvm_apic_get_reg to omit timer cancelation
when mode changes.

timer_mode_mask shouldn't be changing as it depends on cpuid.
Signed-off-by: NRadim Krčmář <rkrcmar@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

APIC-write VM exits are "trap-like": they save CS:RIP values for the
instruction after the write, and more importantly, the handler will
already see the new value in the virtual-APIC page.

This caused a bug if you used KVM_SET_IRQCHIP to set the SW-enabled bit
in the SPIV register.  The chain of events is as follows:

* When the irqchip is added to the destination VM, the apic_sw_disabled
static key is incremented (1)

* When the KVM_SET_IRQCHIP ioctl is invoked, it is decremented (0)

* When the guest disables the bit in the SPIV register, e.g. as part of
shutdown, apic_set_spiv does not notice the change and the static key is
_not_ incremented.

* When the guest is destroyed, the static key is decremented (-1),
resulting in this trace:

  WARNING: at kernel/jump_label.c:81 __static_key_slow_dec+0xa6/0xb0()
  jump label: negative count!

  [<ffffffff816bf898>] dump_stack+0x19/0x1b
  [<ffffffff8107c6f1>] warn_slowpath_common+0x61/0x80
  [<ffffffff8107c76c>] warn_slowpath_fmt+0x5c/0x80
  [<ffffffff811931e6>] __static_key_slow_dec+0xa6/0xb0
  [<ffffffff81193226>] static_key_slow_dec_deferred+0x16/0x20
  [<ffffffffa0637698>] kvm_free_lapic+0x88/0xa0 [kvm]
  [<ffffffffa061c63e>] kvm_arch_vcpu_uninit+0x2e/0xe0 [kvm]
  [<ffffffffa05ff301>] kvm_vcpu_uninit+0x21/0x40 [kvm]
  [<ffffffffa067cec7>] vmx_free_vcpu+0x47/0x70 [kvm_intel]
  [<ffffffffa061bc50>] kvm_arch_vcpu_free+0x50/0x60 [kvm]
  [<ffffffffa061ca22>] kvm_arch_destroy_vm+0x102/0x260 [kvm]
  [<ffffffff810b68fd>] ? synchronize_srcu+0x1d/0x20
  [<ffffffffa06030d1>] kvm_put_kvm+0xe1/0x1c0 [kvm]
  [<ffffffffa06036f8>] kvm_vcpu_release+0x18/0x20 [kvm]
  [<ffffffff81215c62>] __fput+0x102/0x310
  [<ffffffff81215f4e>] ____fput+0xe/0x10
  [<ffffffff810ab664>] task_work_run+0xb4/0xe0
  [<ffffffff81083944>] do_exit+0x304/0xc60
  [<ffffffff816c8dfc>] ? _raw_spin_unlock_irq+0x2c/0x50
  [<ffffffff810fd22d>] ?  trace_hardirqs_on_caller+0xfd/0x1c0
  [<ffffffff8108432c>] do_group_exit+0x4c/0xc0
  [<ffffffff810843b4>] SyS_exit_group+0x14/0x20
  [<ffffffff816d33a9>] system_call_fastpath+0x16/0x1b
Signed-off-by: NRadim Krčmář <rkrcmar@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Expose Intel AVX-512 feature bits to guest. Also add checks for
xcr0 AVX512 related bits according to spec:
http://download-software.intel.com/sites/default/files/managed/71/2e/319433-017.pdfSigned-off-by: NChao Peng <chao.p.peng@linux.intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

The check in kvm_set_lapic_tscdeadline_msr() was trying to prevent a
situation where we lose a pending deadline timer in a MSR write.
Losing it is fine, because it effectively occurs before the timer fired,
so we should be able to cancel or postpone it.

Another problem comes from interaction with QEMU, or other userspace
that can set deadline MSR without a good reason, when timer is already
pending:  one guest's deadline request results in more than one
interrupt because one is injected immediately on MSR write from
userspace and one through hrtimer later.

The solution is to remove the injection when replacing a pending timer
and to improve the usual QEMU path, we inject without a hrtimer when the
deadline has already passed.
Signed-off-by: NRadim Krčmář <rkrcmar@redhat.com>
Reported-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Make the code reusable.

If the timer was already pending, we shouldn't be waiting in a queue,
so wake_up can be skipped, simplifying the path.

There is no 'reinject' case => the comment is removed.
Current race behaves correctly.
Signed-off-by: NRadim Krčmář <rkrcmar@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

If DR4/5 is accessed when it is unavailable (since CR4.DE is set), then #UD
should be generated even if CPL>0. This is according to Intel SDM Table 6-2:
"Priority Among Simultaneous Exceptions and Interrupts".

Note, that this may happen on the first DR access, even if the host does not
sets debug breakpoints. Obviously, it occurs when the host debugs the guest.

This patch moves the DR4/5 checks from __kvm_set_dr/_kvm_get_dr to handle_dr.
The emulator already checks DR4/5 availability in check_dr_read. Nested
virutalization related calls to kvm_set_dr/kvm_get_dr would not like to inject
exceptions to the guest.

As for SVM, the patch follows the previous logic as much as possible. Anyhow,
it appears the DR interception code might be buggy - even if the DR access
may cause an exception, the instruction is skipped.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>