linux-kernel - Re: [PATCH 1/2] KVM: VMX: Use kvm_read

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Date:   Mon, 10 Apr 2023 10:11:06 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: VMX: Use kvm_read_cr4() to get cr4 value

On Mon, Apr 10, 2023, Xiaoyao Li wrote:
> Directly use vcpu->arch.cr4 is not recommended since it gets stale value
> if the cr4 is not available.
> 
> Use kvm_read_cr4() instead to ensure correct value.
> 
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index d7bf14abdba1..befa2486836b 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -3431,7 +3431,7 @@ static bool vmx_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>  
>  void vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>  {
> -	unsigned long old_cr4 = vcpu->arch.cr4;
> +	unsigned long old_cr4 = kvm_read_cr4(vcpu);

Ha!  I've been tempted to change this multiple times, but always thought I was
just being a bit obsessive :-)

Patches look good, but I'm going to hold them for 6.5 just in case this somehow
causes a problem, e.g. if there's a bizzaro nested path that "works" because KVM
_doesn't_ decache info from the current VMCS.