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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v5 01/11] xen/arm: xc_domain_ioport_permission(..) not supported on ARM.
Hi Roger, > 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: >> Hi Roger, >> >>> On 11 Oct 2021, at 14:57, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote: >>> >>> On Mon, Oct 11, 2021 at 01:40:30PM +0000, Bertrand Marquis wrote: >>>> Hi Roger, >>>> >>>> + Oleksandr to have a better PCI expert then me. >>>> >>>>> On 11 Oct 2021, at 14:20, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote: >>>>> >>>>> On Mon, Oct 11, 2021 at 12:11:04PM +0000, Bertrand Marquis wrote: >>>>>> Hi Roger, >>>>>> >>>>>>> On 11 Oct 2021, at 12:47, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote: >>>>>>> >>>>>>> On Wed, Oct 06, 2021 at 06:40:27PM +0100, Rahul Singh wrote: >>>>>>>> ARM architecture does not implement I/O ports. Ignore this call on ARM >>>>>>>> to avoid the overhead of making a hypercall just for Xen to return >>>>>>>> -ENOSYS. >>>>>>> >>>>>>> What is the cal trace of this function actually on Arm? >>>>>>> >>>>>>> AFAICT libxl will only call xc_domain_ioport_permission if there are >>>>>>> IO ports explicitly defined in the guest configuration, or if any of >>>>>>> the BARs of the PCI device is in the IO space, which is not possible >>>>>>> on Arm. >>>>>> >>>>>> PCI devices BARs can be in the IO space as the PCI devices are not >>>>>> Arm specific. There is not ioports on arm so to be used those can be >>>>>> in some cases remapped and accessed as MMIOs or are not possible >>>>>> to use at all. >>>>>> >>>>>> But the IO space does appear when BARs are listed even on Arm. >>>>> >>>>> Urg, I wonder whether those devices with IO BARs will work correctly >>>>> under Arm then. >>>>> >>>>> How do you know whether the BAR has been remapped from IO space into >>>>> MMIO? >>>> >>>> We cannot, I think the platform will define if this is the case and where. >>>> @oleksandr: I remember that this was discussed during some of our >>>> meetings but I have no idea of the details here, can you help ? >>>> >>>>> >>>>> IMO instead of faking a successful return value from >>>>> xc_domain_ioport_permission we should avoid the call completely in the >>>>> first place, specially if we need to instead issue a call to >>>>> xc_domain_iomem_permission. >>>> >>>> At the end we will never have to issue this because this will never be a >>>> matter >>>> of “iomem” permission as there would not be any way to cut on something >>>> under >>>> the page. If this is to be supported one day, it will probably have to be >>>> fully emulated >>>> to keep the isolation. >>> >>> So you have a set of memory pages that map accesses from >>> MMIO into IO space but it's not possible to isolate specific IO port >>> regions as they are all contiguous in the same page(s). >> >> Exact. >> >>> >>>> Right now on arm you can just make the more simple assumption that ioports >>>> are >>>> just not supported. >>> >>> Would it make sense in the future to provide a memory region to guests >>> in order to use for IO port accesses, and call >>> xc_domain_ioport_permission to set which ports would be allowed? >> >> Right now we do not plan to support this at all and we will have to >> figure this out if we do this one day. >> >>> >>> 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. So in the current state I think the way to do it is right. Regards Bertrand > > Thanks, Roger.
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