Re: [PATCH 6/7] x86/cpuid: Fix handling of XSAVE dynamic leaves

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 23/05/2024 5:16 pm, Jan Beulich wrote:
> On 23.05.2024 13:16, Andrew Cooper wrote:
>> First, if XSAVE is available in hardware but not visible to the guest, the
>> dynamic leaves shouldn't be filled in.
>>
>> Second, the comment concerning XSS state is wrong.  VT-x doesn't manage
>> host/guest state automatically, but there is provision for "host only" bits to
>> be set, so the implications are still accurate.
>>
>> Introduce xstate_compressed_size() to mirror the uncompressed one.  Cross
>> check it at boot.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Reviewed-by: Jan Beulich <jbeulich@suse.com>

Thanks.

> Irrespective ...
>
>> v3:
>>  * Adjust commit message about !XSAVE guests
>>  * Rebase over boot time cross check
>>  * Use raw policy
> ... it should probably have occurred to me earlier on to ask: Why raw policy?
> Isn't the host one the more appropriate one to use for any kind of internal
> decisions?

State information is identical in all policies.  It's the ABI of the
X{SAVE,RSTOR}* instructions.

Beyond that, consistency.

xstate_uncompressed_size() does strictly need to be the raw policy,
because it is used by recalculate_xstate() to calculate the host policy.

xstate_compressed_size() doesn't have the same restriction, but should
use the same source of data.

Finally, xstate_{un,}compressed_size() aren't really tied to a choice of
features in the first place.  They shouldn't be limited to the
host_policy's subset of active features.

~Andrew