linux-kernel - Re: [PATCH v4 4/8] KVM: VMX: Handle MMIO Stale Data in VM-Enter assembly via ALTERNATIVES

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Message-ID: <aRTAlEaq-bI5AMFA@google.com>
Date: Wed, 12 Nov 2025 09:15:00 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.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>, 
	Brendan Jackman <jackmanb@...gle.com>
Subject: Re: [PATCH v4 4/8] KVM: VMX: Handle MMIO Stale Data in VM-Enter
 assembly via ALTERNATIVES_2

On Wed, Nov 12, 2025, Borislav Petkov wrote:
> On Thu, Oct 30, 2025 at 05:30:36PM -0700, Sean Christopherson wrote:
> > @@ -137,6 +138,12 @@ SYM_FUNC_START(__vmx_vcpu_run)
> >  	/* Load @regs to RAX. */
> >  	mov (%_ASM_SP), %_ASM_AX
> >  
> > +	/* Stash "clear for MMIO" in EFLAGS.ZF (used below). */
> 
> Oh wow. Alternatives interdependence. What can go wrong. :)

Nothing, it's perfect. :-D

> > +	ALTERNATIVE_2 "",								\
> > +		      __stringify(test $VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO, %ebx), 	\
> 
> So this VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO bit gets set here:
> 
>         if (cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF_MMIO) &&
>             kvm_vcpu_can_access_host_mmio(&vmx->vcpu))
>                 flags |= VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO;
> 
> So how static and/or dynamic is this?

kvm_vcpu_can_access_host_mmio() is very dynamic.  It can be different between
vCPUs in a VM, and can even change on back-to-back runs of the same vCPU.

> 
> IOW, can you stick this into a simple variable which is unconditionally
> updated and you can use it in X86_FEATURE_CLEAR_CPU_BUF_MMIO case and
> otherwise it simply remains unused?

Can you elaborate?  I don't think I follow what you're suggesting.

> 
> Because then you get rid of that yuckiness.
> 
> Thx.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette