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
> 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
>
