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: <20251112164144.GAaRS4yKgF0gQrLSnR@fat_crate.local>
Date: Wed, 12 Nov 2025 17:41:44 +0100
From: Borislav Petkov <bp@...en8.de>
To: Sean Christopherson <seanjc@...gle.com>
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 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. :)

> +	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?

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?

Because then you get rid of that yuckiness.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette