Re: i915 dma faults on Xen

[Jan Beulich <jbeulich@suse.com>]

On 21.10.2020 15:36, Jason Andryuk wrote:
> On Wed, Oct 21, 2020 at 8:53 AM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 21.10.2020 14:45, Jason Andryuk wrote:
>>> On Wed, Oct 21, 2020 at 5:58 AM Roger Pau Monné <roger.pau@citrix.com> 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.

Jan