linux-kernel - Re: [PATCH 2/2] KVM: x86: always expose VIRT

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Date:   Thu, 15 Aug 2019 14:14:54 -0300
From:   Eduardo Habkost <ehabkost@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH 2/2] KVM: x86: always expose VIRT_SSBD to guests

On Thu, Aug 15, 2019 at 09:41:23AM +0200, Paolo Bonzini wrote:
> Even though it is preferrable to use SPEC_CTRL (represented by
> X86_FEATURE_AMD_SSBD) instead of VIRT_SPEC, VIRT_SPEC is always
> supported anyway because otherwise it would be impossible to
> migrate from old to new CPUs.  Make this apparent in the
> result of KVM_GET_SUPPORTED_CPUID as well.
> 
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Reported-by: Eduardo Habkost <ehabkost@...hat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/cpuid.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 145ec050d45d..5865bc73bbb5 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -747,11 +747,13 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
>  		entry->ebx &= kvm_cpuid_8000_0008_ebx_x86_features;
>  		cpuid_mask(&entry->ebx, CPUID_8000_0008_EBX);
>  		/*
> -		 * The preference is to use SPEC CTRL MSR instead of the
> -		 * VIRT_SPEC MSR.
> +		 * VIRT_SPEC is only implemented for AMD processors,
> +		 * but the host could set AMD_SSBD if it wanted even
> +		 * for Intel processors.
>  		 */
> -		if (boot_cpu_has(X86_FEATURE_LS_CFG_SSBD) &&
> -		    !boot_cpu_has(X86_FEATURE_AMD_SSBD))
> +		if ((boot_cpu_has(X86_FEATURE_LS_CFG_SSBD) ||
> +		     boot_cpu_has(X86_FEATURE_AMD_SSBD)) &&
> +		    boot_cpu_has(X86_FEATURE_SVM))

Would it be desirable to move this code to
svm_set_supported_cpuid(), or is there a reason for keeping this
in cpuid.c?


>  			entry->ebx |= F(VIRT_SSBD);
>  		break;
>  	}
> -- 
> 1.8.3.1
> 

-- 
Eduardo