Re: [PATCH v5 2/8] x86/bugs: Allow spectre_v2=ibrs on x86 vendors other than Intel

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

@Sean, @Paolo, Please consider merging the rest of this patchseries 
(patches 3-8), as they can go through the KVM tree directly as-is 
because they does not depend on this or the prior patch 1/8 other than 
for performance reasons.  Thank you!

On 9/29/26 10:06 PM, Borislav Petkov wrote:
> On Fri, Sep 18, 2026 at 06:01:55PM -0500, Kim Phillips wrote:
>> SPEC_CTRL.IBRS is architectural and behaves on AMD just like it does on
>> Intel, but the kernel offers spectre_v2=ibrs on Intel only: the option is
>> rejected at parse time on other vendors, and MITIGATION_IBRS_ENTRY cannot
>> even be built into an AMD-only kernel.
>>
>> AMD's SNP BTB Isolation needs the kernel to toggle legacy IBRS in order to
>> optimize the VM exit-to-re-entry path, and right now there is no way to ask
>> for kernel IBRS on an AMD machine at all.
> Same comments as to the other one: We need:
>
> - a proper use case which is *actually* useful to users

How is optimizing the VM exit-to-re-entry path not "*actually* useful to 
users"?

If BTB Isolation users want to use BTB Isolation, they'd want what the 
AMD APM recommends: To use spectre_v2=ibrs: what is wrong with that?

For this Node-ID getting cpuid script that causes lots of VMEXITS:

for i in \`seq 0 1000\`; do sudo taskset -c 0 cpuid -1 -k -l 0x8000001e 
 > c0; done

I get these stats:

v2=(retpolines), btb-isol=off                   v2=(retpolines), btb-isol=on

8135634850      l1btb-accesses          8237924785 l1btb-accesses        
           101%
2292830105      l1btb-misses            2479438191 l1btb-misses          
           108%
2254370689      l2btb-misses            2462526637 l2btb-misses          
           109%
38479671        l2btb-hits              16912146        l2btb-hits       
               44%
2292846687      l2btb-accesses          2479437240 l2btb-accesses        
           108%

6.591965727     seconds time    6.410626779     seconds time elapsed 97%

v2=ibrs, btb-isol=off                   v2=ibrs, btb-isol=on

9079195678      l1btb-accesses          7735439435 l1btb-accesses        
           85%
3420759821      l1btb-misses            2264575904 l1btb-misses          
           66%
3385686214      l2btb-misses            2226743253 l2btb-misses          
           66%
35073337        l2btb-hits              37834099        l2btb-hits       
               108%
3420753266      l2btb-accesses          2264587491 l2btb-accesses        
           66%

7.019508776     seconds time    6.408415958     seconds time elapsed 91%

Does *that* help?

Kim

>
> - have it properly documented in Documentation/admin-guide/hw-vuln/ along with
>    the potential impact so that people can make an *informed* decision whether
>    they really want it
>
> Without any justification for the added complexity to what already is
> a maintenance nightmare, there's no point.
>
> Thx.
>