Re: [PATCH v3 7/9] xen/arm: vgic: Optimize the way to store the target vCPU in the rank

[Julien Grall <julien.grall@citrix.com>]

On 07/10/15 16:38, Ian Campbell wrote:
> On Wed, 2015-10-07 at 15:41 +0100, Julien Grall wrote:
>> Note that with these changes, any read to those registers will list only
>> the target vCPU used by Xen. We think this is likely to be OK because the
>> GIC spec doesn't require to return exactly the value written and it can 
>> be seen as if we decide to implement the register read-only.
> 
> This still isn't true.

I understood you previous answer on v2 as please replace "this is
fine..." by "We think this is likely to be OK because..."....

> The GIC spec requires one of two modes: Either the register is read-only
> _or_ it is writable and one can expect to read back what was written
> (because the spec doesn't say otherwise and doing otherwise would
> definitely be exceptional behaviour).

How can you say that the spec requires the second mode? Nothing in the
spec say that you can expect to read back what was written...

The spec only says what you the format of the register and what you may
expect to read if the register is read-only. Other than that it's
completely blur and you can implement whatever you want.

Regards,

-- 
Julien Grall