On Thu, May 04, 2017 at 12:58:05PM +0200, Paolo Bonzini wrote:
>
>
> On 03/05/2017 21:37, Radim Krčmář wrote:
> > The guest can call MWAIT with ECX = 0 even if we enforce
> > CPUID5_ECX_INTERRUPT_BREAK; the call would have the exactly the same
> > effect as if the host didn't have CPUID5_ECX_INTERRUPT_BREAK.
> >
> > The check was added in some iteration while trying to fix a reported
> > OS X on Core 2 bug, but the CPU had CPUID5_ECX_INTERRUPT_BREAK and the
> > bug is elsewhere.
>
> The reason for this, as I understood it, is that we have historically
> not published leaf 5 information via KVM_GET_SUPPORTED_CPUID. For this
> reason, QEMU is publishing CPUID5_ECX_INTERRUPT_BREAK. Then if:
>
> - the host doesn't have ECX[0]=1 support
>
> - the guest sets ECX[0]
>
> you get a #GP in the guest. So wrong comment but right thing to do.
>
> Paolo
Exactly. And I agree the comment isn't a good one.
> > Signed-off-by: Radim Krčmář <rkrcmar@xxxxxxxxxx>
> > ---
> > arch/x86/kvm/x86.h | 23 +----------------------
> > 1 file changed, 1 insertion(+), 22 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> > index 63d5fb65ea30..8ea4e80c24d1 100644
> > --- a/arch/x86/kvm/x86.h
> > +++ b/arch/x86/kvm/x86.h
> > @@ -216,8 +216,6 @@ static inline u64 nsec_to_cycles(struct kvm_vcpu *vcpu, u64 nsec)
> >
> > static inline bool kvm_mwait_in_guest(void)
> > {
> > - unsigned int eax, ebx, ecx, edx;
> > -
> > if (!cpu_has(&boot_cpu_data, X86_FEATURE_MWAIT))
> > return false;
> >
> > @@ -225,29 +223,10 @@ static inline bool kvm_mwait_in_guest(void)
> > case X86_VENDOR_AMD:
> > return !boot_cpu_has_bug(X86_BUG_AMD_E400);
> > case X86_VENDOR_INTEL:
> > - /* Handle Intel below */
> > - break;
> > + return !boot_cpu_has_bug(X86_BUG_MONITOR);
> > default:
> > return false;
> > }
> > -
> > - if (boot_cpu_has_bug(X86_BUG_MONITOR))
> > - return false;
> > -
> > - /*
> > - * Intel CPUs without CPUID5_ECX_INTERRUPT_BREAK are problematic as
> > - * they would allow guest to stop the CPU completely by disabling
> > - * interrupts then invoking MWAIT.
> > - */
> > - if (boot_cpu_data.cpuid_level < CPUID_MWAIT_LEAF)
> > - return false;
> > -
> > - cpuid(CPUID_MWAIT_LEAF, &eax, &ebx, &ecx, &edx);
> > -
> > - if (!(ecx & CPUID5_ECX_INTERRUPT_BREAK))
> > - return false;
> > -
> > - return true;
> > }
> >
> > #endif
> >
