Re: [PATCH v4 02/10] cpu/bugs: Allow forcing Automatic IBRS with SNP active using spectre_v2=eibrs

[Borislav Petkov]

On Tue, Aug 04, 2026 at 06:56:03PM -0500, Kim Phillips wrote:

Please fix all your subject prefixes: it should be "x86/bugs:"

The tip tree preferred format for patch subject prefixes is
'subsys/component:', e.g. 'x86/apic:', 'x86/mm/fault:', 'sched/fair:',
'genirq/core:'. Please do not use file names or complete file paths as
prefix. 'git log path/to/file' should give you a reasonable hint in most
cases.

> spectre_v2=eibrs currently enables retpolines when SNP is enabled,
> instead of AutoIBRS (EIBRS) because the commit that disabled
> AutoIBRS if SNP is enabled stopped short of enabling
> X86_FEATURE_IBRS_ENHANCED.
> 
> Change the logic to enable X86_FEATURE_IBRS_ENHANCED, and move the
> decision to switch to retpolines in the default/"auto" case in
> spectre_v2_select_mitigation().  This allows the existing
> spectre_v2=eibrs logic to work as intended.

So this whole hoopla is just to be able to select =eibrs in SNP guests, *and*,
in doing so, cause a performance loss. I.e., shoot oneself in the foot.

Or is there another, better reason which wants this?

> Fixes: acaa4b5c4c85 ("x86/speculation: Do not enable Automatic IBRS if SEV-SNP is enabled")
> Reported-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: Borislav Petkov (AMD) <bp@xxxxxxxxx>
> Cc: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxx

Definitely not stable material. If at all.

> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202603121136.bc8zNsHS-lkp@xxxxxxxxx/

It doesn't close that - that link points to something which is not upstream.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette