Modeled after KVM_CAP_ENFORCE_PV_FEATURE_CPUID, the new capability allows
for limiting Hyper-V features to those exposed to the guest in Hyper-V
CPUIDs (0x40000003, 0x40000004, ...).
Signed-off-by: NVitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210521095204.2161214-3-vkuznets@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

From Hyper-V TLFS:
 "The hypervisor exposes hypercalls (HvFlushVirtualAddressSpace,
  HvFlushVirtualAddressSpaceEx, HvFlushVirtualAddressList, and
  HvFlushVirtualAddressListEx) that allow operating systems to more
  efficiently manage the virtual TLB. The L1 hypervisor can choose to
  allow its guest to use those hypercalls and delegate the responsibility
  to handle them to the L0 hypervisor. This requires the use of a
  partition assist page."

Add the Direct Virtual Flush support for SVM.

Related VMX changes:
commit 6f6a657c ("KVM/Hyper-V/VMX: Add direct tlb flush support")
Signed-off-by: NVineeth Pillai <viremana@linux.microsoft.com>
Message-Id: <fc8d24d8eb7017266bb961e39a171b0caf298d7f.1622730232.git.viremana@linux.microsoft.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Enlightened MSR-Bitmap as per TLFS:

 "The L1 hypervisor may collaborate with the L0 hypervisor to make MSR
  accesses more efficient. It can enable enlightened MSR bitmaps by setting
  the corresponding field in the enlightened VMCS to 1. When enabled, L0
  hypervisor does not monitor the MSR bitmaps for changes. Instead, the L1
  hypervisor must invalidate the corresponding clean field after making
  changes to one of the MSR bitmaps."

Enable this for SVM.

Related VMX changes:
commit ceef7d10 ("KVM: x86: VMX: hyper-v: Enlightened MSR-Bitmap support")
Signed-off-by: NVineeth Pillai <viremana@linux.microsoft.com>
Message-Id: <87df0710f95d28b91cc4ea014fc4d71056eebbee.1622730232.git.viremana@linux.microsoft.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Enable remote TLB flush for SVM.
Signed-off-by: NVineeth Pillai <viremana@linux.microsoft.com>
Message-Id: <1ee364e397e142aed662d2920d198cd03772f1a5.1622730232.git.viremana@linux.microsoft.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

SVM added support for certain reserved fields to be used by
software or hypervisor. Add the following reserved fields:
  - VMCB offset 0x3e0 - 0x3ff
  - Clean bit 31
  - SVM intercept exit code 0xf0000000

Later patches will make use of this for supporting Hyper-V
nested virtualization enhancements.
Signed-off-by: NVineeth Pillai <viremana@linux.microsoft.com>
Message-Id: <a1f17a43a8e9e751a1a9cc0281649d71bdbf721b.1622730232.git.viremana@linux.microsoft.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Currently the remote TLB flush logic is specific to VMX.
Move it to a common place so that SVM can use it as well.
Signed-off-by: NVineeth Pillai <viremana@linux.microsoft.com>
Message-Id: <4f4e4ca19778437dae502f44363a38e99e3ef5d1.1622730232.git.viremana@linux.microsoft.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Bit 22 of HYPERV_CPUID_FEATURES.EDX is specific to SVM and specifies
support for enlightened TLB flush. With this enlightenment enabled,
ASID invalidations flushes only gva->hpa entries. To flush TLB entries
derived from NPT, hypercalls should be used
(HvFlushGuestPhysicalAddressSpace or HvFlushGuestPhysicalAddressList)
Signed-off-by: NVineeth Pillai <viremana@linux.microsoft.com>
Reviewed-by: NMichael Kelley <mikelley@microsoft.com>
Message-Id: <a060f872d0df1955e52e30b877b3300485edb27c.1622730232.git.viremana@linux.microsoft.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Previously, to detect nested virtualization enlightenment support,
we were using HV_X64_ENLIGHTENED_VMCS_RECOMMENDED feature bit of
HYPERV_CPUID_ENLIGHTMENT_INFO.EAX CPUID as docuemented in TLFS:
 "Bit 14: Recommend a nested hypervisor using the enlightened VMCS
  interface. Also indicates that additional nested enlightenments
  may be available (see leaf 0x4000000A)".

Enlightened VMCS, however, is an Intel only feature so the above
detection method doesn't work for AMD. So, use the
HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS.EAX CPUID information ("The
maximum input value for hypervisor CPUID information.") and this
works for both AMD and Intel.
Signed-off-by: NVineeth Pillai <viremana@linux.microsoft.com>
Reviewed-by: NMichael Kelley <mikelley@microsoft.com>
Message-Id: <43b25ff21cd2d9a51582033c9bdd895afefac056.1622730232.git.viremana@linux.microsoft.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Add the following per-VCPU statistic to KVM debugfs to show if a given
VCPU is in guest mode:

	guest_mode

Also add this as a per-VM statistic to KVM debugfs to show the total number
of VCPUs that are in guest mode in a given VM.
Signed-off-by: NKrish Sadhukhan <Krish.Sadhukhan@oracle.com>
Message-Id: <20210609180340.104248-3-krish.sadhukhan@oracle.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Currently, the 'nested_run' statistic counts all guest-entry attempts,
including those that fail during vmentry checks on Intel and during
consistency checks on AMD. Convert this statistic to count only those
guest-entries that make it past these state checks and make it to guest
code. This will tell us the number of guest-entries that actually executed
or tried to execute guest code.
Signed-off-by: NKrish Sadhukhan <Krish.Sadhukhan@oracle.com>
Message-Id: <20210609180340.104248-2-krish.sadhukhan@oracle.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Now that .post_leave_smm() is gone, drop "pre_" from the remaining
helpers.  The helpers aren't invoked purely before SMI/RSM processing,
e.g. both helpers are invoked after state is snapshotted (from regs or
SMRAM), and the RSM helper is invoked after some amount of register state
has been stuffed.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20210609185619.992058-10-seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Drop the .post_leave_smm() emulator callback, which at this point is just
a wrapper to kvm_mmu_reset_context().  The manual context reset is
unnecessary, because unlike enter_smm() which calls vendor MSR/CR helpers
directly, em_rsm() bounces through the KVM helpers, e.g. kvm_set_cr4(),
which are responsible for processing side effects.  em_rsm() is already
subtly relying on this behavior as it doesn't manually do
kvm_update_cpuid_runtime(), e.g. to recognize CR4.OSXSAVE changes.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20210609185619.992058-9-seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Rename the SMM tracepoint, which handles both entering and exiting SMM,
from kvm_enter_smm to kvm_smm_transition.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20210609185619.992058-8-seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Invoke the "entering SMM" tracepoint from kvm_smm_changed() instead of
enter_smm(), effectively moving it from before reading vCPU state to
after reading state (but still before writing it to SMRAM!).  The primary
motivation is to consolidate code, but calling the tracepoint from
kvm_smm_changed() also makes its invocation consistent with respect to
SMI and RSM, and with respect to KVM_SET_VCPU_EVENTS (which previously
only invoked the tracepoint when forcing the vCPU out of SMM).
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20210609185619.992058-7-seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Move the core of SMM hflags modifications into kvm_smm_changed() and use
kvm_smm_changed() in enter_smm().  Clear HF_SMM_INSIDE_NMI_MASK for
leaving SMM but do not set it for entering SMM.  If the vCPU is executing
outside of SMM, the flag should unequivocally be cleared, e.g. this
technically fixes a benign bug where the flag could be left set after
KVM_SET_VCPU_EVENTS, but the reverse is not true as NMI blocking depends
on pre-SMM state or userspace input.

Note, this adds an extra kvm_mmu_reset_context() to enter_smm().  The
extra/early reset isn't strictly necessary, and in a way can never be
necessary since the vCPU/MMU context is in a half-baked state until the
final context reset at the end of the function.  But, enter_smm() is not
a hot path, and exploding on an invalid root_hpa is probably better than
having a stale SMM flag in the MMU role; it's at least no worse.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20210609185619.992058-6-seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Move RSM emulation's call to kvm_smm_changed() from .post_leave_smm() to
.exiting_smm(), leaving behind the MMU context reset.  The primary
motivation is to allow for future cleanup, but this also fixes a bug of
sorts by queueing KVM_REQ_EVENT even if RSM causes shutdown, e.g. to let
an INIT wake the vCPU from shutdown.  Of course, KVM doesn't properly
emulate a shutdown state, e.g. KVM doesn't block SMIs after shutdown, and
immediately exits to userspace, so the event request is a moot point in
practice.

Moving kvm_smm_changed() also moves the RSM tracepoint.  This isn't
strictly necessary, but will allow consolidating the SMI and RSM
tracepoints in a future commit (by also moving the SMI tracepoint).
Invoking the tracepoint before loading SMRAM state also means the SMBASE
that reported in the tracepoint will point that the state that will be
used for RSM, as opposed to the SMBASE _after_ RSM completes, which is
arguably a good thing if the tracepoint is being used to debug a RSM/SMM
issue.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20210609185619.992058-5-seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Replace the .set_hflags() emulator hook with a dedicated .exiting_smm(),
moving the SMM and SMM_INSIDE_NMI flag handling out of the emulator in
the process.  This is a step towards consolidating much of the logic in
kvm_smm_changed(), including the SMM hflags updates.

No functional change intended.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20210609185619.992058-4-seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Use the recently introduced KVM_REQ_TRIPLE_FAULT to properly emulate
shutdown if RSM from SMM fails.

Note, entering shutdown after clearing the SMM flag and restoring NMI
blocking is architecturally correct with respect to AMD's APM, which KVM
also uses for SMRAM layout and RSM NMI blocking behavior.  The APM says:

  An RSM causes a processor shutdown if an invalid-state condition is
  found in the SMRAM state-save area. Only an external reset, external
  processor-initialization, or non-maskable external interrupt (NMI) can
  cause the processor to leave the shutdown state.

Of note is processor-initialization (INIT) as a valid shutdown wake
event, as INIT is blocked by SMM, implying that entering shutdown also
forces the CPU out of SMM.

For recent Intel CPUs, restoring NMI blocking is technically wrong, but
so is restoring NMI blocking in the first place, and Intel's RSM
"architecture" is such a mess that just about anything is allowed and can
be justified as micro-architectural behavior.

Per the SDM:

  On Pentium 4 and later processors, shutdown will inhibit INTR and A20M
  but will not change any of the other inhibits. On these processors,
  NMIs will be inhibited if no action is taken in the SMI handler to
  uninhibit them (see Section 34.8).

where Section 34.8 says:

  When the processor enters SMM while executing an NMI handler, the
  processor saves the SMRAM state save map but does not save the
  attribute to keep NMI interrupts disabled. Potentially, an NMI could be
  latched (while in SMM or upon exit) and serviced upon exit of SMM even
  though the previous NMI handler has still not completed.

I.e. RSM unconditionally unblocks NMI, but shutdown on RSM does not,
which is in direct contradiction of KVM's behavior.  But, as mentioned
above, KVM follows AMD architecture and restores NMI blocking on RSM, so
that micro-architectural detail is already lost.

And for Pentium era CPUs, SMI# can break shutdown, meaning that at least
some Intel CPUs fully leave SMM when entering shutdown:

  In the shutdown state, Intel processors stop executing instructions
  until a RESET#, INIT# or NMI# is asserted.  While Pentium family
  processors recognize the SMI# signal in shutdown state, P6 family and
  Intel486 processors do not.

In other words, the fact that Intel CPUs have implemented the two
extremes gives KVM carte blanche when it comes to honoring Intel's
architecture for handling shutdown during RSM.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20210609185619.992058-3-seanjc@google.com>
[Return X86EMUL_CONTINUE after triple fault. - Paolo]
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Now that APICv/AVIC enablement is kept in common 'enable_apicv' variable,
there's no need to call kvm_apicv_init() from vendor specific code.

No functional change intended.
Reviewed-by: NSean Christopherson <seanjc@google.com>
Reviewed-by: NMaxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: NVitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210609150911.1471882c-3-vkuznets@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Unify VMX and SVM code by moving APICv/AVIC enablement tracking to common
'enable_apicv' variable. Note: unlike APICv, AVIC is disabled by default.

No functional change intended.
Suggested-by: NSean Christopherson <seanjc@google.com>
Signed-off-by: NVitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210609150911.1471882c-2-vkuznets@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Implement PM hibernation/suspend prepare notifiers so that KVM
can reliably set PVCLOCK_GUEST_STOPPED on VCPUs and properly
suspend VMs.
Signed-off-by: NSergey Senozhatsky <senozhatsky@chromium.org>
Message-Id: <20210606021045.14159-2-senozhatsky@chromium.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Don't allow posted interrupts to modify a stale posted interrupt
descriptor (including the initial value of 0).

Empirical tests on real hardware reveal that a posted interrupt
descriptor referencing an unbacked address has PCI bus error semantics
(reads as all 1's; writes are ignored). However, kvm can't distinguish
unbacked addresses from device-backed (MMIO) addresses, so it should
really ask userspace for an MMIO completion. That's overly
complicated, so just punt with KVM_INTERNAL_ERROR.

Don't return the error until the posted interrupt descriptor is
actually accessed. We don't want to break the existing kvm-unit-tests
that assume they can launch an L2 VM with a posted interrupt
descriptor that references MMIO space in L1.

Fixes: 6beb7bd5 ("kvm: nVMX: Refactor nested_get_vmcs12_pages()")
Signed-off-by: NJim Mattson <jmattson@google.com>
Message-Id: <20210604172611.281819-8-jmattson@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

When the kernel has no mapping for the vmcs02 virtual APIC page,
userspace MMIO completion is necessary to process nested posted
interrupts. This is not a configuration that KVM supports. Rather than
silently ignoring the problem, try to exit to userspace with
KVM_INTERNAL_ERROR.

Note that the event that triggers this error is consumed as a
side-effect of a call to kvm_check_nested_events. On some paths
(notably through kvm_vcpu_check_block), the error is dropped. In any
case, this is an incremental improvement over always ignoring the
error.
Signed-off-by: NJim Mattson <jmattson@google.com>
Message-Id: <20210604172611.281819-7-jmattson@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

No functional change intended. At present, the only negative value
returned by kvm_check_nested_events is -EBUSY.
Signed-off-by: NJim Mattson <jmattson@google.com>
Message-Id: <20210604172611.281819-6-jmattson@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

No functional change intended. At present, 'r' will always be -EBUSY
on a control transfer to the 'out' label.
Signed-off-by: NJim Mattson <jmattson@google.com>
Message-Id: <20210604172611.281819-5-jmattson@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

No functional change intended.
Signed-off-by: NJim Mattson <jmattson@google.com>
Reviewed-by: NOliver Upton <oupton@google.com>
Message-Id: <20210604172611.281819-4-jmattson@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

A survey of the callsites reveals that they all ensure the vCPU is in
guest mode before calling kvm_check_nested_events. Remove this dead
code so that the only negative value this function returns (at the
moment) is -EBUSY.
Signed-off-by: NJim Mattson <jmattson@google.com>
Message-Id: <20210604172611.281819-2-jmattson@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Calculate the TSC offset and multiplier on nested transitions and expose
the TSC scaling feature to L1.
Signed-off-by: NIlias Stamatis <ilstam@amazon.com>
Reviewed-by: NMaxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210526184418.28881-11-ilstam@amazon.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Currently vmx_vcpu_load_vmcs() writes the TSC_MULTIPLIER field of the
VMCS every time the VMCS is loaded. Instead of doing this, set this
field from common code on initialization and whenever the scaling ratio
changes.

Additionally remove vmx->current_tsc_ratio. This field is redundant as
vcpu->arch.tsc_scaling_ratio already tracks the current TSC scaling
ratio. The vmx->current_tsc_ratio field is only used for avoiding
unnecessary writes but it is no longer needed after removing the code
from the VMCS load path.
Suggested-by: NSean Christopherson <seanjc@google.com>
Signed-off-by: NIlias Stamatis <ilstam@amazon.com>
Message-Id: <20210607105438.16541-1-ilstam@amazon.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

The write_l1_tsc_offset() callback has a misleading name. It does not
set L1's TSC offset, it rather updates the current TSC offset which
might be different if a nested guest is executing. Additionally, both
the vmx and svm implementations use the same logic for calculating the
current TSC before writing it to hardware.

Rename the function and move the common logic to the caller. The vmx/svm
specific code now merely sets the given offset to the corresponding
hardware structure.
Signed-off-by: NIlias Stamatis <ilstam@amazon.com>
Reviewed-by: NMaxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210526184418.28881-9-ilstam@amazon.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

When L2 is entered we need to "merge" the TSC multiplier and TSC offset
values of 01 and 12 together.

The merging is done using the following equations:
  offset_02 = ((offset_01 * mult_12) >> shift_bits) + offset_12
  mult_02 = (mult_01 * mult_12) >> shift_bits

Where shift_bits is kvm_tsc_scaling_ratio_frac_bits.
Signed-off-by: NIlias Stamatis <ilstam@amazon.com>
Reviewed-by: NMaxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210526184418.28881-8-ilstam@amazon.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

In order to implement as much of the nested TSC scaling logic as
possible in common code, we need these vendor callbacks for retrieving
the TSC offset and the TSC multiplier that L1 has set for L2.
Signed-off-by: NIlias Stamatis <ilstam@amazon.com>
Reviewed-by: NMaxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210526184418.28881-7-ilstam@amazon.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

This is required for supporting nested TSC scaling.
Signed-off-by: NIlias Stamatis <ilstam@amazon.com>
Reviewed-by: NJim Mattson <jmattson@google.com>
Reviewed-by: NMaxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210526184418.28881-6-ilstam@amazon.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Sometimes kvm_scale_tsc() needs to use the current scaling ratio and
other times (like when reading the TSC from user space) it needs to use
L1's scaling ratio. Have the caller specify this by passing the ratio as
a parameter.
Signed-off-by: NIlias Stamatis <ilstam@amazon.com>
Reviewed-by: NMaxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210526184418.28881-5-ilstam@amazon.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

All existing code uses kvm_compute_tsc_offset() passing L1 TSC values to
it. Let's document this by renaming it to kvm_compute_l1_tsc_offset().
Signed-off-by: NIlias Stamatis <ilstam@amazon.com>
Reviewed-by: NMaxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210526184418.28881-4-ilstam@amazon.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Store L1's scaling ratio in the kvm_vcpu_arch struct like we already do
for L1's TSC offset. This allows for easy save/restore when we enter and
then exit the nested guest.
Signed-off-by: NIlias Stamatis <ilstam@amazon.com>
Reviewed-by: NMaxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210526184418.28881-3-ilstam@amazon.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

If the TDP MMU is in use, wait to allocate the rmaps until the shadow
MMU is actually used. (i.e. a nested VM is launched.) This saves memory
equal to 0.2% of guest memory in cases where the TDP MMU is used and
there are no nested guests involved.
Signed-off-by: NBen Gardon <bgardon@google.com>
Message-Id: <20210518173414.450044-8-bgardon@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

If only the TDP MMU is being used to manage the memory mappings for a VM,
then many rmap operations can be skipped as they are guaranteed to be
no-ops. This saves some time which would be spent on the rmap operation.
It also avoids acquiring the MMU lock in write mode for many operations.

This makes it safe to run the VM without rmaps allocated, when only
using the TDP MMU and sets the stage for waiting to allocate the rmaps
until they're needed.
Signed-off-by: NBen Gardon <bgardon@google.com>
Message-Id: <20210518173414.450044-7-bgardon@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Add a field to control whether new memslots should have rmaps allocated
for them. As of this change, it's not safe to skip allocating rmaps, so
the field is always set to allocate rmaps. Future changes will make it
safe to operate without rmaps, using the TDP MMU. Then further changes
will allow the rmaps to be allocated lazily when needed for nested
oprtation.

No functional change expected.
Reviewed-by: NDavid Hildenbrand <david@redhat.com>
Signed-off-by: NBen Gardon <bgardon@google.com>
Message-Id: <20210518173414.450044-6-bgardon@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Small refactor to facilitate allocating rmaps for all memslots at once.

No functional change expected.
Signed-off-by: NBen Gardon <bgardon@google.com>
Message-Id: <20210518173414.450044-3-bgardon@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>