linux-kernel - Re: [PATCH v5 4/5] KVM: VMX: wrap guest access to IA32

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Message-ID: <aC9jPd25Jf2vU5h7@google.com>
Date: Thu, 22 May 2025 10:47:41 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: kvm@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org, 
	x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v5 4/5] KVM: VMX: wrap guest access to IA32_DEBUGCTL with wrappers

This doesn't just wrap guest access, it wraps all access.

On Wed, May 21, 2025, Maxim Levitsky wrote:
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 00f2b762710c..b505f3f7e9ab 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2653,7 +2653,7 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
>  	if (vmx->nested.nested_run_pending &&
>  	    (vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS)) {
>  		kvm_set_dr(vcpu, 7, vmcs12->guest_dr7);
> -		vmcs_write64(GUEST_IA32_DEBUGCTL, vmcs12->guest_ia32_debugctl);
> +		vmx_guest_debugctl_write(vcpu, vmcs12->guest_ia32_debugctl);
>  	} else {
>  		kvm_set_dr(vcpu, 7, vcpu->arch.dr7);
>  		vmcs_write64(GUEST_IA32_DEBUGCTL, vmx->nested.pre_vmenter_debugctl);

I think it makes sense to use the accessors for this case as well.  Conceptually,
pre_vmenter_debugctl holds the guest value.  The fact that it holds the combined
value _as written_ is rather subtle, and could change for the worse, e.g. it'd
be quite unfortunate/hilarious if someone converted the read path to use the
getter, and in doing so introduced a bug.