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

Nikolay Borisov
2025-11-21T14:27:08Z

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
additionally you duplicate the FEATURE_IBPB check.

So I think either dropping it or removing the superfluous branches is in
order.

> else
> vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
> + break;
> }
> }
>
>