Re: [PATCH 2/4] x86/P2M: relax guarding of MMIO entries

[Jan Beulich <jbeulich@xxxxxxxx>]

On 31.08.2021 17:38, Jan Beulich wrote:
> On 31.08.2021 17:25, Andrew Cooper wrote:
>> On 31/08/2021 14:26, Jan Beulich wrote:
>>> On 31.08.2021 15:16, Andrew Cooper wrote:
>>>> On 30/08/2021 14:02, Jan Beulich wrote:
>>>>> Further permit "access" to differ in the "executable" attribute. While
>>>>> ideally only ROM regions would get mapped with X set, getting there is
>>>>> quite a bit of work. Therefore, as a temporary measure, permit X to
>>>>> vary. For Dom0 the more permissive of the types will be used, while for
>>>>> DomU it'll be the more restrictive one.
>>>> Split behaviour between dom0 and domU based on types alone cannot
>>>> possibly be correct.
>>> True, but what do you do.
>>>
>>>> DomU's need to execute ROMs too, and this looks like will malfunction if
>>>> a ROM ends up in the region that HVMLoader relocated RAM from.
>>>>
>>>> As this is a temporary bodge emergency bugfix, don't try to be clever -
>>>> just take the latest access.
>>> And how do we know that that's what is going to work?
>>
>> Because it's the pre-existing behaviour.
> 
> Valid point. But for the DomU case there simply has not been any
> pre-existing behavior. Hence my desire to be restrictive initially
> there.

Further to this: Using the last-value-set approach also puts us at
risk of running into a similar issue again when the ordering of
some operations changes elsewhere.

Jan