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". Jan
