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