On 07/05/2024 3:24 pm, Roger Pau Monné wrote: > 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 <[email protected]> > Acked-by: Roger Pau Monné <[email protected]>
Thanks. > > 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? This is the final max leaf (containing feature information) to gain custom handling, I think? ~Andrew
