Re: [PATCH v5 01/11] xen/arm: xc_domain_ioport_permission(..) not supported on ARM.

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Jan,

> On 12 Oct 2021, at 10:32, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> 
> On 12.10.2021 10:41, Bertrand Marquis wrote:
>> Hi Jan,
>> 
>>> On 12 Oct 2021, at 09:29, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>> 
>>> On 11.10.2021 19:11, Bertrand Marquis wrote:
>>>>> On 11 Oct 2021, at 17:32, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
>>>>> On Mon, Oct 11, 2021 at 02:16:19PM +0000, Bertrand Marquis wrote:
>>>>>>> On 11 Oct 2021, at 14:57, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
>>>>>>> I think the commit message needs to at least be expanded in order to
>>>>>>> contain the information provided here. It might also be helpful to
>>>>>>> figure out whether we would have to handle IO port accesses in the
>>>>>>> future on Arm, or if it's fine to just ignore them.
>>>>>> 
>>>>>> All our investigations and tests have been done without supporting it
>>>>>> without any issues so this is not a critical feature (most devices can
>>>>>> be operated without using the I/O ports).
>>>>> 
>>>>> IMO we should let the users know they attempted to use a device with
>>>>> BARs in the IO space, and that those BARs won't be accessible which
>>>>> could make the device not function as expected.
>>>>> 
>>>>> Do you think it would be reasonable to attempt the hypercall on Arm
>>>>> also, and in case of error (on Arm) just print a warning message and
>>>>> continue operations as normal?
>>>> 
>>>> I think this would lead to a warning printed on lots of devices where in
>>>> fact there would be no issues.
>>>> 
>>>> If this is an issue for a device driver because it cannot operate without
>>>> I/O ports, this will be raised by the driver inside the guest.
>>> 
>>> On what basis would the driver complain? The kernel might know of
>>> the MMIO equivalent for ports, and hence might allow the driver
>>> to properly obtain whatever is needed to later access the ports.
>>> Just that the port accesses then wouldn't work (possibly crashing
>>> the guest, or making it otherwise misbehave).
>> 
>> As ECAM and Arm does not support I/O ports, a driver requesting access
>> to them would get an error back.
>> So in practice it is not possible to try to access the ioports as there is no
>> way on arm to use them (no instructions).
>> 
>> A driver could misbehave by ignoring the fact that ioports are not there but
>> I am not quite sure how we could solve that as it would be a bug in the 
>> driver.
> 
> The minimal thing I'd suggest (or maybe you're doing this already)
> would be to expose such BARs to the guest as r/o zero, rather than
> letting their port nature "shine through".

We are emulating an ECAM PCI which does not support I/O ports so I do not
think we are (and can) expose those to guests.

Anyway I will mark this as a point to check for Rahul when he is back.

Cheers
Bertrand

> 
> Jan