linux-kernel - Re: [PATCH AUTOSEL 5.3 28/68] KVM: x86: Expose XSAVEERPTR to the guest

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Message-ID: <05acd554-dd0a-d7cd-e17c-90627fa0ec67@redhat.com>
Date:   Wed, 9 Oct 2019 23:15:07 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        kvm@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 5.3 28/68] KVM: x86: Expose XSAVEERPTR to the
 guest

On 09/10/19 19:05, Sasha Levin wrote:
> From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> 
> [ Upstream commit 504ce1954fba888936c9d13ccc1e3db9b8f613d5 ]
> 
> I was surprised to see that the guest reported `fxsave_leak' while the
> host did not. After digging deeper I noticed that the bits are simply
> masked out during enumeration.
> 
> The XSAVEERPTR feature is actually a bug fix on AMD which means the
> kernel can disable a workaround.
> 
> Pass XSAVEERPTR to the guest if available on the host.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  arch/x86/kvm/cpuid.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index fd1b8db8bf242..59b66e343fa5a 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -479,6 +479,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
>  
>  	/* cpuid 0x80000008.ebx */
>  	const u32 kvm_cpuid_8000_0008_ebx_x86_features =
> +		F(XSAVEERPTR) |
>  		F(WBNOINVD) | F(AMD_IBPB) | F(AMD_IBRS) | F(AMD_SSBD) | F(VIRT_SSBD) |
>  		F(AMD_SSB_NO) | F(AMD_STIBP) | F(AMD_STIBP_ALWAYS_ON);
>  
> 

Yet another example of a patch that shouldn't be stable material (in
this case it's fine, but there can certainly be cases where just adding
a single flag depends on core kernel changes).

Paolo