RE: [PATCH v5 03/16] x86/bugs: Restructure MMIO mitigation

Kaplan, David
2025-04-24T20:31:29Z

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Borislav Petkov <bp@alien8.de>
> Sent: Thursday, April 24, 2025 4:19 PM
> To: Kaplan, David <David.Kaplan@amd.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>; Peter Zijlstra <peterz@infradead.org>;
> Josh Poimboeuf <jpoimboe@kernel.org>; Pawan Gupta
> <pawan.kumar.gupta@linux.intel.com>; Ingo Molnar <mingo@redhat.com>; Dave
> Hansen <dave.hansen@linux.intel.com>; x86@kernel.org; H . Peter Anvin
> <hpa@zytor.com>; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH v5 03/16] x86/bugs: Restructure MMIO mitigation
>
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>
>
> On Fri, Apr 18, 2025 at 11:17:08AM -0500, David Kaplan wrote:
> > +static void __init mmio_apply_mitigation(void) {
> > + if (mmio_mitigation == MMIO_MITIGATION_OFF)
> > + return;
> >
> > /*
> > - * X86_FEATURE_CLEAR_CPU_BUF could be enabled by other VERW based
> > - * mitigations, disable KVM-only mitigation in that case.
> > + * Only enable the VMM mitigation if the CPU buffer clear mitigation is
> > + * not being used.
> > */
> > - if (boot_cpu_has(X86_FEATURE_CLEAR_CPU_BUF))
> > + if (verw_mitigation_selected) {
> > + setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
> > static_branch_disable(&cpu_buf_vm_clear);
> > - else
> > + } else {
> > static_branch_enable(&cpu_buf_vm_clear);
> > + }
>
> Sorry, but I'm still not happy about this.
>
> After this patch, we have:
>
> /*
> * Enable CPU buffer clear mitigation for host and VMM, if also affected
> * by MDS or TAA.
> */
> if (boot_cpu_has_bug(X86_BUG_MDS) || taa_vulnerable())
> verw_mitigation_selected = true;
>
> in the select function.
>
> The comment is wrong. The code does: enable the VERW mitigation for MMIO if
> affected by MDS or TAA. verw_mitigation_selected doesn't have any bearing on
> whether this should be a host or VMM mitigation - as its name says, a VERW
> mitigation has been selected.

verw_mitigation_selected implies that X86_FEATURE_CLEAR_CPU_BUF will be enabled, which does a VERW on kernel/vmm exits.

So I'm not sure the comment is really wrong, but it can be rephrased.

>
> Then in the apply function:
>
> /*
> * Only enable the VMM mitigation if the CPU buffer clear mitigation is
> * not being used.
> */
> if (verw_mitigation_selected) {
> setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
> static_branch_disable(&cpu_buf_vm_clear);
> } else {
> static_branch_enable(&cpu_buf_vm_clear);
> }
>
> Comment is again wrong. verw_mitigation_selected doesn't mean the CPU buffer
> clear mitigation is not being used.

But it kind of does. !verw_mitigation_selected means that the X86_FEATURE bit there isn't set. So the VMM-based mitigation (the static branch) is only used if the broader X86_FEATURE_CLEAR_CPU_BUF is not being used.

>
> Yes yes, it boils down to the same thing in the end but reading it confusing as hell.
> verw_mitigation_selected means what its name is: a VERW mitigation has been
> selected and nothing else.
>
> Renaming the var is probably unnecessary churn but you can fix the comments and
> still rename the key:
>
> ---
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index
> c97ded4d55e5..4a5bd6214508 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -157,8 +157,8 @@
> DEFINE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
> * X86_FEATURE_CLEAR_CPU_BUF, and should only be enabled when KVM-only
> * mitigation is required.
> */
> -DEFINE_STATIC_KEY_FALSE(cpu_buf_vm_clear);
> -EXPORT_SYMBOL_GPL(cpu_buf_vm_clear);
> +DEFINE_STATIC_KEY_FALSE(clear_cpu_buf_vm);
> +EXPORT_SYMBOL_GPL(clear_cpu_buf_vm);
>
> void __init cpu_select_mitigations(void) { @@ -528,10 +528,7 @@ static void
> __init mmio_select_mitigation(void)
> if (mmio_mitigation == MMIO_MITIGATION_OFF)
> return;
>
> - /*
> - * Enable CPU buffer clear mitigation for host and VMM, if also affected
> - * by MDS or TAA.
> - */
> + /* Enable full VERW mitigation if also affected by MDS or TAA.
> + */
> if (boot_cpu_has_bug(X86_BUG_MDS) || taa_vulnerable())
> verw_mitigation_selected = true; } @@ -568,14 +565,14 @@ static void
> __init mmio_apply_mitigation(void)
> return;
>
> /*
> - * Only enable the VMM mitigation if the CPU buffer clear mitigation is
> - * not being used.
> + * Full VERW mitigation selection enables host and VMENTER buffer clearing,
> + * otherwise buffer clearing only on VMENTER.
> */
> if (verw_mitigation_selected) {
> setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
> - static_branch_disable(&cpu_buf_vm_clear);
> + static_branch_disable(&clear_cpu_buf_vm);
> } else {
> - static_branch_enable(&cpu_buf_vm_clear);
> + static_branch_enable(&clear_cpu_buf_vm);
> }
>
> /*
> @@ -681,7 +678,7 @@ static void __init md_clear_update_mitigation(void)
> taa_select_mitigation();
> }
> /*
> - * MMIO_MITIGATION_OFF is not checked here so that cpu_buf_vm_clear
> + * MMIO_MITIGATION_OFF is not checked here so that
> + clear_cpu_buf_vm
> * gets updated correctly as per X86_FEATURE_CLEAR_CPU_BUF state.
> */
> if (boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA)) {
> ---
>

I'm ok with this patch, as long as 'full VERW mitigation' is considered a clear enough term. I think the updated comment in the apply function does explain what that means, so if that's good enough I'm ok.

--David Kaplan