linux-kernel - Re: [PATCH] KVM: x86: Update the states size cpuid even if XCR0/IA32

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Message-ID: <YecHK2DmooVlMr2U@google.com>
Date:   Tue, 18 Jan 2022 18:30:03 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Update the states size cpuid even if
 XCR0/IA32_XSS is reset

On Mon, Jan 17, 2022, Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> XCR0 is reset to 1 by RESET but not INIT and IA32_XSS is zeroed by
> both RESET and INIT. In both cases, the size in bytes of the XSAVE
> area containing all states enabled by XCR0 or (XCRO | IA32_XSS)
> needs to be updated.
> 
> Fixes: a554d207dc46 ("KVM: X86: Processor States following Reset or INIT")
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
>  arch/x86/kvm/x86.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 76b4803dd3bd..5748a57e1cb7 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11134,6 +11134,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  	struct kvm_cpuid_entry2 *cpuid_0x1;
>  	unsigned long old_cr0 = kvm_read_cr0(vcpu);
>  	unsigned long new_cr0;
> +	bool need_update_cpuid = false;
>  
>  	/*
>  	 * Several of the "set" flows, e.g. ->set_cr0(), read other registers
> @@ -11199,6 +11200,8 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  
>  		vcpu->arch.msr_misc_features_enables = 0;
>  
> +		if (vcpu->arch.xcr0 != XFEATURE_MASK_FP)
> +			need_update_cpuid = true;
>  		vcpu->arch.xcr0 = XFEATURE_MASK_FP;
>  	}
>  
> @@ -11216,6 +11219,8 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  	cpuid_0x1 = kvm_find_cpuid_entry(vcpu, 1, 0);
>  	kvm_rdx_write(vcpu, cpuid_0x1 ? cpuid_0x1->eax : 0x600);
>  
> +	if (vcpu->arch.ia32_xss)
> +		need_update_cpuid = true;

This means that kvm_set_msr_common()'s handling of MSR_IA32_XSS also needs to
update kvm_update_cpuid_runtime().  And then for bnoth XCR0 and XSS, I would very
strongly prefer that use the helpers to write the values and let the helpers call
kvm_update_cpuid_runtime().  Yes, that will mean kvm_update_cpuid_runtime() may be
called multiple times during INIT, but that's already true (CR4), and this isn't
exactly a fast path.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 55518b7d3b96..22d4b1d15e94 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11256,7 +11256,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)

                vcpu->arch.msr_misc_features_enables = 0;

-               vcpu->arch.xcr0 = XFEATURE_MASK_FP;
+               __kvm_set_xcr(vcpu, 0, XFEATURE_MASK_FP);
        }

        /* All GPRs except RDX (handled below) are zeroed on RESET/INIT. */
@@ -11273,7 +11273,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
        cpuid_0x1 = kvm_find_cpuid_entry(vcpu, 1, 0);
        kvm_rdx_write(vcpu, cpuid_0x1 ? cpuid_0x1->eax : 0x600);

-       vcpu->arch.ia32_xss = 0;
+       __kvm_set_msr(vcpu, MSR_IA32_XSS, 0, true);

        static_call(kvm_x86_vcpu_reset)(vcpu, init_event);