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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Nov 25, 2021 at 11:25:36AM +0100, Jan Beulich wrote:
> 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.

I'm certainly happy to look at newer versions of this patch, but I
think we should consider doing the shrinking only on the toolstack
said, and only after all the manipulations on the policy have been
performed.

Thanks, Roger.