Re: [PATCH v3 10/20] i386/cpu: Add missing migratable xsave features

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 10/30/2025 11:56 PM, Zhao Liu wrote:
>> can you elaborate what will be broken without the patch?
> 
> Obviously, the migratable_flags has been broken.
> 
>> As I commented earlier, though the .migrable_flags determines the return
>> value of x86_cpu_get_supported_feature_word() for
>> features[FEAT_XSAVE_XCR0_LO] in x86_cpu_expand_features(), eventually the
>> x86_cpu_enable_xsave_components() overwrites features[FEAT_XSAVE_XCR0_LO].
>> So even we set the migratable_flags to 0 for FEAT_XSAVE_XCR0_LO, it doesn't
>> have any issue.
> 
> No. this seemingly 'correct' result what you see is just due to
> different bugs influencing each other: the flags are wrong, the code
> path is wrong, and the impact produced by the flags is also wrong.
> 
>> So I think we can remove migratable_flags totally.
> 
> migratable_flags is used for feature leaves that are non-migratable by
> default, while unmigratable_flags is used for feature leaves that are
> migratable by default. Simply removing it doesn't need much effort, but
> additional clarification is needed - about whether and how it affects
> the migratable/unmigratable feature setting. Therefore, it's better to
> do such refactor in the separate thread rather than combining it with
> CET for now.

I meant we don't need to bother touching .migratable_flags field as this 
patch because it doesn't make any difference to the functionality. Just 
drop it from the series and feel free to clean it up with a separate series.

> Regards,
> Zhao
> 
>