Re: [RFC PATCH v2 10/11] hw/arm/smmuv3: Populate OAS based on CPU PARANGE

[Mostafa Saleh]

Hi Peter,

On Tue, Mar 21, 2023 at 01:34:55PM +0000, Peter Maydell wrote:
> > >>> +    s->idr[5] = FIELD_DP32(s->idr[5], IDR5, OAS, oas);
> > >> I am not sure you can change that easily. In case of migration this is
> > >> going to change the behavior of the device, no?
> > > I see IDR registers are not migrated. I guess we can add them in a
> > > subsection and if they were not passed (old instances) we set OAS to
> > > 44.
> > > Maybe this should be another change outside of this series.
> > Indeed tehy are not migrated so it can lead to inconsistent behavior in
> > both source and dest. This deserves more analysis to me. In case you
> > would decide to migrate IDR regs this would need to be done in that
> > series I think. Migration must not be broken by this series.
> 
> Jumping in here without having read much of the context, but why
> would we need to migrate the ID registers? They are constant, read-only,
> so they will be the same value on both source and destination.

Currently OAS for SMMU is hardcoded to 44 bits, and the SMMU manual says
"OAS reflects the maximum usable PA output from the last stage of
AArch64 translations, and must match the system physical address size.
The OAS is discoverable from SMMU_IDR5.OAS."
This patch implements OAS based on CPU PARANGE, but this would break
migration from old instances that ran with 44 bits OAS to new code that
configures it based on the current CPU.
So one idea is to migrate the IDRs (or atleast IDR5).
Maybe that is not the best solution, we just need a way to know if the
old instance needs to be 44 bits or not.


Thanks,
Mostafa