Re: [PATCH v2 5/7] xen/arm: Use sanitize values for p2m

[Julien Grall <julien@xxxxxxx>]

On 23/08/2021 13:39, Bertrand Marquis wrote:
> Hi Julien,
Hi Bertrand,

> > On 23 Aug 2021, at 12:47, Julien Grall <julien@xxxxxxx> wrote:
> >
> > Hi Bertrand,
> >
> > On 23/08/2021 11:32, Bertrand Marquis wrote:
> > > Replace the code in p2m trying to find a sane value for the VMID size
> > > supported and the PAR to use. We are now using the boot cpuinfo as the
> > > values there are sanitized during boot and the value for those
> > > parameters is now the safest possible value on the system.
> > > On arm32, the system will panic if there are different types of core so
> > > those checks were not needed anyway.
> > So the assumption is that if you have the same MIDR, then you must have the 
> > same features. I understand this is what Xen assumes today but I never viewed 
> > that check as the truth. It is more to limit the damage on most platform.
> This was the assumption before, I did not change anything but just explained in 
> the commit message why this was not possible to come to this code.
Yes. This is not a new, however I thought I would mention it because I 
want to avoid continuing to use wrong assumptions.
However, this code is arm64 only as it is #ifdef right? (Sorry I should 
have looked at the code the first time) So ...
> > So can you confirm whether this is something that Arm guarantees?
> For a specific MIDR from Arm (ie a Cortex) the PAR is fixed and VMID size to.
> But for an other Arm architecture processor I cannot say.
>
> > That said, I am not against removing the code. But I would like the comment to 
> > be amended if this is not a correct assumption.
> Would the following be acceptable:
> On arm32, Xen is not booting on systems having different MIDR amongst cores and 
> it is assumed that cores with the same MIDR will have the same PAR and VMID 
> size.
... I would just drop any mention of arm32 here.

Cheers,

--
Julien Grall