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

Nikolay Borisov
2025-11-25T10:19:35Z

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@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.

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;
>>> }
>>> }
>>>
>>