linux-kernel - Re: [PATCH 2/2] KVM: x86: Expose CLZERO and XSAVEERPTR to the guest

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Date:   Thu, 26 Sep 2019 13:12:25 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH 2/2] KVM: x86: Expose CLZERO and XSAVEERPTR to the guest

On 25/09/19 23:37, Sebastian Andrzej Siewior wrote:
> 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.
> While here, I've seen that CLZERO is also masked out. This opcode is
> unprivilged so exposing it to the guest should not make any difference.
> 
> Pass CLZERO and XSAVEERPTR to the guest if available on the host.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  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 22c2720cd948e..0ae9194d0f4d2 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -473,6 +473,7 @@ 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(CLZERO) | 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);
>  
> 

Applied (on top of Jim's CLZERO patch, so removing the CLZERO reference
in the commit message).

Paolo