linux-kernel - Re: [PATCH 1/5] KVM: x86: fix presentation of TSX feature in ARCH

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Date:   Fri, 22 Nov 2019 12:15:38 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        jmattson@...gle.com, stable@...r.kernel.org
Subject: Re: [PATCH 1/5] KVM: x86: fix presentation of TSX feature in
 ARCH_CAPABILITIES

On Mon, Nov 18, 2019 at 07:17:43PM +0100, Paolo Bonzini wrote:
> KVM does not implement MSR_IA32_TSX_CTRL, so it must not be presented
> to the guests.  It is also confusing to have !ARCH_CAP_TSX_CTRL_MSR &&
> !RTM && ARCH_CAP_TAA_NO: lack of MSR_IA32_TSX_CTRL suggests TSX was not
> hidden (it actually was), yet the value says that TSX is not vulnerable
> to microarchitectural data sampling.  Fix both.
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5d530521f11d..6ea735d632e9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1327,12 +1327,18 @@ static u64 kvm_get_arch_capabilities(void)
>  	 * If TSX is disabled on the system, guests are also mitigated against
>  	 * TAA and clear CPU buffer mitigation is not required for guests.
>  	 */
> -	if (boot_cpu_has_bug(X86_BUG_TAA) && boot_cpu_has(X86_FEATURE_RTM) &&
> -	    (data & ARCH_CAP_TSX_CTRL_MSR))
> +	if (!boot_cpu_has(X86_FEATURE_RTM))
> +		data &= ~ARCH_CAP_TAA_NO;
> +	else if (!boot_cpu_has_bug(X86_BUG_TAA))
> +		data |= ARCH_CAP_TAA_NO;
> +	else if (data & ARCH_CAP_TSX_CTRL_MSR)
>  		data &= ~ARCH_CAP_MDS_NO;
>  
> +	/* KVM does not emulate MSR_IA32_TSX_CTRL.  */
> +	data &= ~ARCH_CAP_TSX_CTRL_MSR;
>  	return data;
>  }
> +EXPORT_SYMBOL_GPL(kvm_get_arch_capabilities);

Whoever backports this patch should drop this spurious addition of
EXPORT_SYMBOL_GPL, unless they also want to backport the cleanup :-).