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

[Jan Beulich <jbeulich@suse.com>]

On 12.10.2021 11:38, Oleksandr Andrushchenko wrote:
> On 12.10.21 12:32, Jan Beulich wrote:
>> On 12.10.2021 10:41, Bertrand Marquis wrote:
>>>> 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".
> If we have the same, but baremetal then which entity disallows
> those BARs to shine?

I'm sorry, but I don't understand what you're trying to say.

> I mean that if guest wants to crash... why
> should we stop it and try emulating something special for it?

This isn't about a guest "wanting to crash", but a driver potentially
getting mislead into thinking that it can driver a device a certain
way.

Jan