Re: i915 dma faults on Xen

[Jason Andryuk <jandryuk@xxxxxxxxx>]

On Wed, Oct 21, 2020 at 9:59 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 21.10.2020 15:36, Jason Andryuk wrote:
> > On Wed, Oct 21, 2020 at 8:53 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
> >>
> >> On 21.10.2020 14:45, Jason Andryuk wrote:
> >>> On Wed, Oct 21, 2020 at 5:58 AM Roger Pau Monné <roger.pau@xxxxxxxxxx> 
> >>> wrote:
> >>>> Hm, it's hard to tell what's going on. My limited experience with
> >>>> IOMMU faults on broken systems there's a small range that initially
> >>>> triggers those, and then the device goes wonky and starts accessing a
> >>>> whole load of invalid addresses.
> >>>>
> >>>> You could try adding those manually using the rmrr Xen command line
> >>>> option [0], maybe you can figure out which range(s) are missing?
> >>>
> >>> They seem to change, so it's hard to know.  Would there be harm in
> >>> adding one to cover the end of RAM ( 0x04,7c80,0000 ) to (
> >>> 0xff,ffff,ffff )?  Maybe that would just quiet the pointless faults
> >>> while leaving the IOMMU enabled?
> >>
> >> While they may quieten the faults, I don't think those faults are
> >> pointless. They indicate some problem with the software (less
> >> likely the hardware, possibly the firmware) that you're using.
> >> Also there's the question of what the overall behavior is going
> >> to be when devices are permitted to access unpopulated address
> >> ranges. I assume you did check already that no devices have their
> >> BARs placed in that range?
> >
> > Isn't no-igfx already letting them try to read those unpopulated addresses?
>
> Yes, and it is for the reason that the documentation for the
> option says "If specifying `no-igfx` fixes anything, please
> report the problem." I imply from in in particular that one
> better wouldn't use it for non-development purposes of whatever
> kind.

I stopped seeing these DMA faults, but I didn't know what made them go
away.  Then when working with an older 5.4.64 kernel, I saw them
again.  Eric bisected down to the 5.4.y version of mainline linux
commit:

commit 8195400f7ea95399f721ad21f4d663a62c65036f
Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Date:   Mon Oct 19 11:15:23 2020 +0100

    drm/i915: Force VT'd workarounds when running as a guest OS

    If i915.ko is being used as a passthrough device, it does not know if
    the host is using intel_iommu. Mixing the iommu and gfx causes a few
    issues (such as scanout overfetch) which we need to workaround inside
    the driver, so if we detect we are running under a hypervisor, also
    assume the device access is being virtualised.

Regards,
Jason