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

[Jan Beulich <jbeulich@suse.com>]

On 12.10.2021 10:41, Bertrand Marquis wrote:
> Hi Jan,
> 
>> On 12 Oct 2021, at 09:29, Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 11.10.2021 19:11, Bertrand Marquis wrote:
>>>> On 11 Oct 2021, at 17:32, Roger Pau Monné <roger.pau@citrix.com> 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@citrix.com> 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".

Jan