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

  • To: Julien Grall <julien@xxxxxxx>
  • From: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
  • Date: Mon, 23 Aug 2021 12:39:10 +0000
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  • Thread-topic: [PATCH v2 5/7] xen/arm: Use sanitize values for p2m

Hi Julien,

> 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.

> 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 


> Cheers,
> -- 
> Julien Grall



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