Re: [PATCH v3 1/6] cpu/bugs: Allow forcing Automatic IBRS with SNP active using spectre_v2=eibrs

[Kim Phillips <kim.phillips@amd.com>]

On 4/28/26 11:49 AM, Pawan Gupta wrote:
> On Thu, Apr 02, 2026 at 03:25:53PM -0500, Kim Phillips wrote:
>> 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
> 
> The retpoline switch happens in force case(=on) too.

That's right, for default/"=auto"/"=on", if SNP is enabled, retpolines.
If SNP is not enabled, AutoIBRS.  I'm assuming that's the desired
behaviour.

I'll amend the commit text in the next version if more reasons
arise to submit one.

>> spectre_v2_select_mitigation().  This allows the existing
>> spectre_v2=eibrs logic to work as intended.
>>
>> Also emit a performance loss warning for using AutoIBRS with
>> SNP enabled.
>>
>> Fixes: acaa4b5c4c85 ("x86/speculation: Do not enable Automatic IBRS if SEV-SNP is enabled")
>> Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
>> Cc: Borislav Petkov (AMD) <bp@alien8.de>
>> Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>> Cc: Sean Christopherson <seanjc@google.com>
>> Cc: stable@kernel.org
>> Reported-by: kernel test robot <lkp@intel.com>
>> Closes: https://lore.kernel.org/oe-kbuild-all/202603121136.bc8zNsHS-lkp@intel.com/
>> Signed-off-by: Kim Phillips <kim.phillips@amd.com>
>> ---
> 
> Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>

Thanks,

Kim