linux-kernel - Re: [PATCH v4 0/8] x86/bugs: KVM: L1TF and MMIO Stale Data cleanups

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Message-ID: <aQTzlivZDrT_tZRL@google.com>
Date: Fri, 31 Oct 2025 10:36:22 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Brendan Jackman <jackmanb@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Subject: Re: [PATCH v4 0/8] x86/bugs: KVM: L1TF and MMIO Stale Data cleanups

On Fri, Oct 31, 2025, Brendan Jackman wrote:
> On Fri Oct 31, 2025 at 12:30 AM UTC, Sean Christopherson wrote:
> > This is a combination of Brendan's work to unify the L1TF L1D flushing
> > mitigation, and Pawan's work to bring some sanity to the mitigations that
> > clear CPU buffers, with a bunch of glue code and some polishing from me.
> >
> > The "v4" is relative to the L1TF series.  I smushed the two series together
> > as Pawan's idea to clear CPU buffers for MMIO in vmenter.S obviated the need
> > for a separate cleanup/fix to have vmx_l1d_flush() return true/false, and
> > handling the series separately would have been a lot of work+churn for no
> > real benefit.
> >
> > TL;DR:
> >
> >  - Unify L1TF flushing under per-CPU variable
> >  - Bury L1TF L1D flushing under CONFIG_CPU_MITIGATIONS=y
> >  - Move MMIO Stale Data into asm, and do VERW at most once per VM-Enter
> >
> > To allow VMX to use ALTERNATIVE_2 to select slightly different flows for doing
> > VERW, tweak the low lever macros in nospec-branch.h to define the instruction
> > sequence, and then wrap it with __stringify() as needed.
> >
> > The non-VMX code is lightly tested (but there's far less chance for breakage
> > there).  For the VMX code, I verified it does what I want (which may or may
> > not be correct :-D) by hacking the code to force/clear various mitigations, and
> > using ud2 to confirm the right path got selected.
> 
> FWIW [0] offers a way to check end-to-end that an L1TF exploit is broken
> by the mitigation. It's a bit of a long-winded way to achieve that and I
> guess L1TF is anyway the easy case here, but I couldn't resist promoting
> it.

Yeah, it's on my radar, but it'll be a while before I have the bandwidth to dig
through something that involved (though I _am_ excited to have a way to actually
test mitigations).