linux-kernel - Re: [PATCH] KVM: x86: SVM: Mark VMCB_LBR dirty when L1 sets DebugCtl[LBR]

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Message-ID: <tavsqj24pscngpu5pxfuvpsylcn72anoc7q5i5goip5lb5fqpt@xh2srbojpvfs>
Date: Mon, 3 Nov 2025 17:42:03 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Jim Mattson <jmattson@...gle.com>
Cc: Sean Christopherson <seanjc@...gle.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, 
	Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Matteo Rizzo <matteorizzo@...gle.com>, evn@...gle.com
Subject: Re: [PATCH] KVM: x86: SVM: Mark VMCB_LBR dirty when L1 sets
 DebugCtl[LBR]

On Fri, Oct 31, 2025 at 05:02:29PM -0700, Jim Mattson wrote:
> With the VMCB's LBR_VIRTUALIZATION_ENABLE bit set, the CPU will load
> the DebugCtl MSR from the VMCB's DBGCTL field at VMRUN. To ensure that
> it does not load a stale cached value, clear the VMCB's LBR clean bit
> when L1 is running and bit 0 (LBR) of the DBGCTL field is changed from
> 0 to 1. (Note that this is already handled correctly when L2 is
> running.)
> 
> There is no need to clear the clean bit in the other direction,
> because when the VMCB's DBGCTL.LBR is 0, the VMCB's
> LBR_VIRTUALIZATION_ENABLE bit will be clear, and the CPU will not
> consult the VMCB's DBGCTL field at VMRUN.

Is it worth the mental load of figuring out why we do it in
svm_enable_lbrv() but not svm_disable_lbrv()?

Maybe we can at least document it in svm_disable_lbrv() with a comment?

> 
> Fixes: 1d5a1b5860ed ("KVM: x86: nSVM: correctly virtualize LBR msrs when L2 is running")
> Reported-by: Matteo Rizzo <matteorizzo@...gle.com>
> Reported-by: evn@...gle.com
> Signed-off-by: Jim Mattson <jmattson@...gle.com>
> ---
>  arch/x86/kvm/svm/svm.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 153c12dbf3eb..b4e5a0684f57 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -816,6 +816,8 @@ void svm_enable_lbrv(struct kvm_vcpu *vcpu)
>  	/* Move the LBR msrs to the vmcb02 so that the guest can see them. */
>  	if (is_guest_mode(vcpu))
>  		svm_copy_lbrs(svm->vmcb, svm->vmcb01.ptr);
> +	else
> +		vmcb_mark_dirty(svm->vmcb, VMCB_LBR);
>  }
>  
>  static void svm_disable_lbrv(struct kvm_vcpu *vcpu)
> -- 
> 2.51.2.1006.ga50a493c49-goog
>