Re: [PATCH for-4.16] Revert "x86/CPUID: shrink max_{,sub}leaf fields according to actual leaf contents"

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.11.2021 22:11, Andrew Cooper wrote:
> OSSTest has identified a 3rd regression caused by this change.  Migration
> between Xen 4.15 and 4.16 on the nocera pair of machines (AMD Opteron 4133)
> fails with:
> 
>   xc: error: Failed to set CPUID policy: leaf 00000000, subleaf ffffffff, msr 
> ffffffff (22 = Invalid argument): Internal error
>   xc: error: Restore failed (22 = Invalid argument): Internal error
> 
> which is a safety check to prevent resuming the guest when the CPUID data has
> been truncated.  The problem is caused by shrinking of the max policies, which
> is an ABI that needs handling compatibly between different versions of Xen.
> 
> Furthermore, shrinking of the default policies also breaks things in some
> cases, because certain cpuid= settings in a VM config file which used to have
> an effect will now be silently discarded.
> 
> This reverts commit 540d911c2813c3d8f4cdbb3f5672119e5e768a3d, as well as the
> partial fix attempt in 81da2b544cbb003a5447c9b14d275746ad22ab37 (which added
> one new case where cpuid= settings might not apply correctly) and restores the
> same behaviour as Xen 4.15.
> 
> Fixes: 540d911c2813 ("x86/CPUID: shrink max_{,sub}leaf fields according to 
> actual leaf contents")
> Fixes: 81da2b544cbb ("x86/cpuid: prevent shrinking migrated policies max 
> leaves")
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

While not strictly needed with Roger having given his already,
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
to signal my (basic) agreement with the course of action taken.
Nevertheless I fear this is going to become yet one more case where
future action is promised, but things then die out.

Ian - I guess all this now needs is your R-a.

Jan