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

Pawan Gupta
2025-11-24T23:09:23Z

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@linux.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;
> > }
> > }
> >
>