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

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Message-ID: <CALMp9eTvkg0JB4gOniUrPJ1-Xc4van3+DSco676UuC6ZY4PjLw@mail.gmail.com>
Date: Mon, 3 Nov 2025 10:10:02 -0800
From: Jim Mattson <jmattson@...gle.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
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 Mon, Nov 3, 2025 at 9:42 AM Yosry Ahmed <yosry.ahmed@...ux.dev> wrote:
>
> 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?

I'm happy to do it in svm_disable_lbrv() as well, just to reduce the
cognitive load.