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    SubjectRe: [PATCH v3] kvm: better MWAIT emulation for guests
    2017-03-14 01:44+0200, Michael S. Tsirkin:
    > Guests running Mac OS 5, 6, and 7 (Leopard through Lion) have a problem:
    > unless explicitly provided with kernel command line argument
    > "idlehalt=0" they'd implicitly assume MONITOR and MWAIT availability,
    > without checking CPUID.
    >
    > We currently emulate that as a NOP but on VMX we can do better: let
    > guest stop the CPU until timer, IPI or memory change. CPU will be busy
    > but that isn't any worse than a NOP emulation.
    >
    > Note that mwait within guests is not the same as on real hardware
    > because halt causes an exit while mwait doesn't. For this reason it
    > might not be a good idea to use the regular MWAIT flag in CPUID to
    > signal this capability. Add a flag in the hypervisor leaf instead.
    >
    > Additionally, we add a capability for QEMU - e.g. if it knows there's an
    > isolated CPU dedicated for the VCPU it can set the standard MWAIT flag
    > to improve guest behaviour.
    >
    > Reported-by: "Gabriel L. Somlo" <gsomlo@gmail.com>
    > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
    > ---
    >
    > Note: SVM bits are untested at this point. Seems pretty
    > obvious though.
    >
    > changes from v2:
    > - add a capability to allow host userspace to detect new kernels
    > - more documentation to clarify the semantics of the feature flag
    > and why it's useful
    > - svm support as suggested by Radim
    >
    > changes from v1:
    > - typo fix resulting in rest of leaf flags being overwritten
    > Reported by: Wanpeng Li <kernellwp@gmail.com>
    > - updated commit log with data about guests helped by this feature
    > - better document differences between mwait and halt for guests
    >
    > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
    > @@ -4135,11 +4135,11 @@ available, means that that the kernel can support guests using the
    > radix MMU defined in Power ISA V3.00 (as implemented in the POWER9
    > processor).
    >
    > -8.4 KVM_CAP_PPC_HASH_MMU_V3

    This patch should not not remove the PPC capability from docs.

    (The right name is KVM_CAP_PPC_HASH_V3, but that is for another patch.)

    > +8.5 KVM_CAP_X86_GUEST_MWAIT
    >
    > -Architectures: ppc
    > +Architectures: x86
    >
    > -This capability, if KVM_CHECK_EXTENSION indicates that it is
    > -available, means that that the kernel can support guests using the
    > -hashed page table MMU defined in Power ISA V3.00 (as implemented in
    > -the POWER9 processor), including in-memory segment tables.
    > +This capability indicates that guest using memory monotoring instructions
    > +(MWAIT/MWAITX) to stop the virtual CPU will not cause a VM exit. As such time
    > +spent while virtual CPU is halted in this way will then be accounted for as
    > +guest running time on the host (as opposed to e.g. HLT).
    > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
    > @@ -2684,6 +2684,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
    > case KVM_CAP_ADJUST_CLOCK:
    > r = KVM_CLOCK_TSC_STABLE;
    > break;
    > + case KVM_CAP_X86_GUEST_MWAIT:
    > + r = !!this_cpu_has(X86_FEATURE_MWAIT);

    this_cpu_has already returns bool, so !! is not needed.

    I can fix both while applying.

    > + break;
    > case KVM_CAP_X86_SMM:
    > /* SMBASE is usually relocated above 1M on modern chipsets,
    > * and SMM handlers might indeed rely on 4G segment limits,

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