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    SubjectRe: [PATCH 1/3] KVM: X86: Provides userspace with a capability to not intercept MWAIT
    2018-03-09 4:31 GMT+08:00 Radim Krčmář <rkrcmar@redhat.com>:
    >> To check if a capability can be enabled, the KVM_CHECK_EXTENSION ioctl should
    >> -be used.
    >> +be used. Blindly passing the KVM_CHECK_EXTENSION result to KVM_ENABLE_CAP is
    >> +a valid thing to do when vCPUs are associated to dedicated physical CPUs.
    >
    > This is not true even for x86 KVM_CAP_SPLIT_IRQCHIP and neither is is a
    > need to limit ourselves. Just leave it be.

    https://www.spinics.net/lists/kvm/msg159524.html

    > So I think we should put in the
    > documentation that blindly passing the KVM_CHECK_EXTENSION result to
    > KVM_ENABLE_CAP is a valid thing to do when vCPUs are associated to
    > dedicated physical CPUs.

    Paolo ask this before, Paolo, what's your opinion?

    >> +7.13 KVM_CAP_X86_DISABLE_EXITS
    >> +
    >> +Architectures: x86
    >> +Parameters: args[0] defines which exits are disabled
    >> +Returns: 0 on success, -EINVAL when args[0] contains invalid exits
    >> +
    >> +Valid exits in args[0] are
    >> +
    >> +#define KVM_X86_DISABLE_EXITS_MWAIT (1 << 0)
    >> +
    >> +Enabling this capability on a VM provides userspace with a way to no
    >> +longer intercepts some instructions for improved latency in some
    >> +workloads. Not enable KVM_FEATURE_PV_UNHALT if you block HLT.
    >
    > The last sentence belong to the patch that enables HLT.
    > KVM could in theory handle the case (although it makes no sense), so if

    > it doesn't currently work, please add a check to kvm_update_cpuid() that
    > forbids KVM_FEATURE_PV_UNHALT when halt exits are disabled.

    Agreed.

    >
    > Also, it would be nicer to write that as
    > "Do not enable KVM_FEATURE_PV_UNHALT if you disable HLT exits."
    >
    >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
    >> @@ -2780,9 +2780,15 @@ static int msr_io(struct kvm_vcpu *vcpu, struct kvm_msrs __user *user_msrs,
    >> return r;
    >> }
    >>
    >> +static inline bool kvm_mwait_can_in_guest(void)
    >
    > I think kvm_can_mwait_in_guest would be a better name.

    Agreed.

    Regards,
    Wanpeng Li

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