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

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Message-ID: <6f749888-28ef-419b-bc0a-5a82b6b58787@amd.com>
Date: Thu, 6 Nov 2025 21:38:54 +0530
From: Shivansh Dhiman <shivansh.dhiman@....com>
To: Jim Mattson <jmattson@...gle.com>
CC: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
	<pbonzini@...hat.com>, <nikunj.dadhania@....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 01-11-2025 05:32, 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.
> 
> 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)

Hi Jim,
I am thinking, is it possible to add a test in KVM Unit Tests that
covers this? Something where the stale cached value is loaded instead of
the correct one, without your patch.

Best regards,
Shivansh