linux-kernel - Re: [PATCH v4 06/11] x86/vmscape: Move mitigation selection to a switch()

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Message-ID: <20251124230917.7wxvux5s6j6f5tuz@desk>
Date: Mon, 24 Nov 2025 15:09:17 -0800
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Nikolay Borisov <nik.borisov@...e.com>
Cc: x86@...nel.org, David Kaplan <david.kaplan@....com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Asit Mallick <asit.k.mallick@...el.com>,
	Tao Zhang <tao1.zhang@...el.com>
Subject: Re: [PATCH v4 06/11] x86/vmscape: Move mitigation selection to a
 switch()

On Fri, Nov 21, 2025 at 04:27:05PM +0200, Nikolay Borisov wrote:
> 
> 
> On 11/20/25 08:19, Pawan Gupta wrote:
> > This ensures that all mitigation modes are explicitly handled, while
> > keeping the mitigation selection for each mode together. This also prepares
> > for adding BHB-clearing mitigation mode for VMSCAPE.
> > 
> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> > ---
> >   arch/x86/kernel/cpu/bugs.c | 22 ++++++++++++++++++----
> >   1 file changed, 18 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index 1e9b11198db0fe2483bd17b1327bcfd44a2c1dbf..233594ede19bf971c999f4d3cc0f6f213002c16c 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -3231,17 +3231,31 @@ early_param("vmscape", vmscape_parse_cmdline);
> >   static void __init vmscape_select_mitigation(void)
> >   {
> > -	if (!boot_cpu_has_bug(X86_BUG_VMSCAPE) ||
> > -	    !boot_cpu_has(X86_FEATURE_IBPB)) {
> > +	if (!boot_cpu_has_bug(X86_BUG_VMSCAPE)) {
> >   		vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
> >   		return;
> >   	}
> > -	if (vmscape_mitigation == VMSCAPE_MITIGATION_AUTO) {
> > -		if (should_mitigate_vuln(X86_BUG_VMSCAPE))
> > +	if ((vmscape_mitigation == VMSCAPE_MITIGATION_AUTO) &&
> > +	    !should_mitigate_vuln(X86_BUG_VMSCAPE))
> > +		vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
> > +
> > +	switch (vmscape_mitigation) {
> > +	case VMSCAPE_MITIGATION_NONE:
> > +		break;
> > +
> > +	case VMSCAPE_MITIGATION_IBPB_ON_VMEXIT:
> > +	case VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER:
> > +		if (!boot_cpu_has(X86_FEATURE_IBPB))
> > +			vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
> > +		break;
> > +
> > +	case VMSCAPE_MITIGATION_AUTO:
> > +		if (boot_cpu_has(X86_FEATURE_IBPB))
> >   			vmscape_mitigation = VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER;
> 
> 
> IMO this patch is a net-negative because as per my reply to patch 9 you have
> effectively a dead branch:
> 
> The clear BHB_CLEAR_USER one, however it turns out you have yet another one:
> VMSCAPE_MITIGATION_IBPB_ON_VMEXIT as it's only ever set in
> vmscape_update_mitigation() which executes after '_select()' as well and

Removed VMSCAPE_MITIGATION_IBPB_ON_VMEXIT.

> additionally you duplicate the FEATURE_IBPB check.

FEATURE_IBPB check is still needed for VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER.
I don't think we can drop that.

> So I think either dropping it or removing the superfluous branches is in
> order.
> 
> >   		else
> >   			vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
> > +		break;
> >   	}
> >   }
> > 
>