Re: [PATCH v2] x86/cpu-policy: Fix migration from Ice Lake to Cascade Lake

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

On Tue, May 07, 2024 at 02:45:40PM +0100, Andrew Cooper wrote:
> Ever since Xen 4.14, there has been a latent bug with migration.
> 
> While some toolstacks can level the features properly, they don't shink
> feat.max_subleaf when all features have been dropped.  This is because
> we *still* have not completed the toolstack side work for full CPU Policy
> objects.
> 
> As a consequence, even when properly feature levelled, VMs can't migrate
> "backwards" across hardware which reduces feat.max_subleaf.  One such example
> is Ice Lake (max_subleaf=2 for INTEL_PSFD) to Cascade Lake (max_subleaf=0).
> 
> Extend the max policies feat.max_subleaf to the hightest number Xen knows
> about, but leave the default policies matching the host.  This will allow VMs
> with a higher feat.max_subleaf than strictly necessary to migrate in.
> 
> Eventually we'll manage to teach the toolstack how to avoid creating such VMs
> in the first place, but there's still more work to do there.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Even if we have just found one glitch with PSFD and Ice Lake vs
Cascade Lack, wouldn't it be safer to always extend the max policies
max leafs and subleafs to match the known array sizes?

Thanks, Roger.