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

[Nikolay Borisov]

On 11/25/25 01:09, Pawan Gupta wrote:
> 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@xxxxxxxxxxxxxxx>
> > > ---
> > >    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.

But if X86_FEATURE_IBPB is not present then all branches boil down to 
setting the mitigation to NONE. What I was suggesting is to not remove 
the that check at the top.

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