linux-kernel - Re: [PATCH v2] KVM: x86: Revise guest_fpu xcomp

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Date:   Mon, 1 Mar 2021 15:59:59 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jing Liu <jing2.liu@...ux.intel.com>
Cc:     pbonzini@...hat.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>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86: Revise guest_fpu xcomp_bv field

On Thu, Feb 25, 2021, Jing Liu wrote:
> XCOMP_BV[63] field indicates that the save area is in the compacted
> format and XCOMP_BV[62:0] indicates the states that have space allocated
> in the save area, including both XCR0 and XSS bits enabled by the host
> kernel. Use xfeatures_mask_all for calculating xcomp_bv and reuse
> XCOMP_BV_COMPACTED_FORMAT defined by kernel.
> 
> Signed-off-by: Jing Liu <jing2.liu@...ux.intel.com>
> ---
>  arch/x86/kvm/x86.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1b404e4d7dd8..f115493f577d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4435,8 +4435,6 @@ static int kvm_vcpu_ioctl_x86_set_debugregs(struct kvm_vcpu *vcpu,
>  	return 0;
>  }
>  
> -#define XSTATE_COMPACTION_ENABLED (1ULL << 63)
> -
>  static void fill_xsave(u8 *dest, struct kvm_vcpu *vcpu)
>  {
>  	struct xregs_state *xsave = &vcpu->arch.guest_fpu->state.xsave;
> @@ -4494,7 +4492,8 @@ static void load_xsave(struct kvm_vcpu *vcpu, u8 *src)
>  	/* Set XSTATE_BV and possibly XCOMP_BV.  */
>  	xsave->header.xfeatures = xstate_bv;
>  	if (boot_cpu_has(X86_FEATURE_XSAVES))
> -		xsave->header.xcomp_bv = host_xcr0 | XSTATE_COMPACTION_ENABLED;
> +		xsave->header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT |
> +					 xfeatures_mask_all;

Doesn't fill_xsave also need to be updated?  Not with xfeatures_mask_all, but
to account for arch.ia32_xss?  I believe it's a nop with the current code, since
supported_xss is zero, but it should be fixed, no?

>  
>  	/*
>  	 * Copy each region from the non-compacted offset to the
> @@ -9912,9 +9911,6 @@ static void fx_init(struct kvm_vcpu *vcpu)
>  		return;
>  
>  	fpstate_init(&vcpu->arch.guest_fpu->state);
> -	if (boot_cpu_has(X86_FEATURE_XSAVES))
> -		vcpu->arch.guest_fpu->state.xsave.header.xcomp_bv =
> -			host_xcr0 | XSTATE_COMPACTION_ENABLED;

Ugh, this _really_ needs a comment in the changelog.  It took me a while to
realize fpstate_init() does exactly what the new fill_xave() is doing.

And isn't the code in load_xsave() redundant and can be removed?  Any code that
uses get_xsave_addr() would be have a dependency on load_xsave() if it's not
redundant, and I can't see how that would work.

>  
>  	/*
>  	 * Ensure guest xcr0 is valid for loading
> -- 
> 2.18.4
>