linux-kernel - Re: [PATCH 3/4] KVM: x86: drop bogus MWAIT check

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Date:   Thu, 4 May 2017 12:58:05 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Alexander Graf <agraf@...e.de>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "Gabriel L. Somlo" <gsomlo@...il.com>
Subject: Re: [PATCH 3/4] KVM: x86: drop bogus MWAIT check



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@...hat.com>
> ---
>  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
>