linux-kernel - Re: [PATCH v4 1/8] x86/bugs: Use VM_CLEAR_CPU

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Message-ID: <DDWH24WOQG3F.1VS7MT0SKPWIL@google.com>
Date: Fri, 31 Oct 2025 11:30:54 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, 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>
Cc: <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 1/8] x86/bugs: Use VM_CLEAR_CPU_BUFFERS in VMX as well

Rewording my comments from:
https://lore.kernel.org/all/20251029-verw-vm-v1-1-babf9b961519@linux.intel.com/

On Fri Oct 31, 2025 at 12:30 AM UTC, Sean Christopherson wrote:
> From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
>
> TSA mitigation:
>
>   d8010d4ba43e ("x86/bugs: Add a Transient Scheduler Attacks mitigation")
>
> introduced VM_CLEAR_CPU_BUFFERS for guests on AMD CPUs. Currently on Intel
> CLEAR_CPU_BUFFERS is being used for guests which has a much broader scope
> (kernel->user also).
>
> Make mitigations on Intel consistent with TSA. This would help handling the
> guest-only mitigations better in future.
>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> [sean: make CLEAR_CPU_BUF_VM mutually exclusive with the MMIO mitigation]
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

I think this is a clear improvement. Now that X86_FEATURE_CLEAR_CPU_BUF
has a clear scope, can we also update the comment on its definition in
cpufeatures.h? I.e. say that it's specifically about exit to user.

This also seems like a good moment to update the comment on
verw_clear_cpu_buf_mitigation_selected to mention the _VM flag too.

Also, where we set vmx->disable_fb_clear in vmx_update_fb_clear_dis(),
it still refers to X86_FEATURE_CLEAR_CPU_BUF, is that wrong?