linux-kernel - Re: [PATCH v2 04/10] KVM: x86: Remove defunct setting of CR0.ET for guests during vCPU create

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Message-ID: <87zgs680t9.fsf@vitty.brq.redhat.com>
Date:   Tue, 21 Sep 2021 16:23:30 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Reiji Watanabe <reijiw@...gle.com>
Subject: Re: [PATCH v2 04/10] KVM: x86: Remove defunct setting of CR0.ET for
 guests during vCPU create

Sean Christopherson <seanjc@...gle.com> writes:

> Drop code to set CR0.ET for the guest during initialization of the guest
> FPU.  The code was added as a misguided bug fix by commit 380102c8e431
> ("KVM Set the ET flag in CR0 after initializing FX") to resolve an issue
> where vcpu->cr0 (now vcpu->arch.cr0) was not correctly initialized on SVM
> systems.  While init_vmcb() did set CR0.ET, it only did so in the VMCB,
> and subtly did not update vcpu->cr0.  Stuffing CR0.ET worked around the
> immediate problem, but did not fix the real bug of vcpu->cr0 and the VMCB
> being out of sync.  That underlying bug was eventually remedied by commit
> 18fa000ae453 ("KVM: SVM: Reset cr0 properly on vcpu reset").
>
> No functional change intended.

fx_init() is only called from kvm_arch_vcpu_create() (and inlined later
in the series) a few lines before kvm_vcpu_reset() which stuffs CR0 with 
X86_CR0_ET too and it doesn't seem that arch.cr0 value is important in
between.

>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/x86.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ab907a0b9eeb..e0bff5473813 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10628,8 +10628,6 @@ static void fx_init(struct kvm_vcpu *vcpu)
>  	 * Ensure guest xcr0 is valid for loading
>  	 */
>  	vcpu->arch.xcr0 = XFEATURE_MASK_FP;
> -
> -	vcpu->arch.cr0 |= X86_CR0_ET;
>  }
>  
>  void kvm_free_guest_fpu(struct kvm_vcpu *vcpu)

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly