Re: [Xen-devel] Upstream Dom0 DRM problems regarding swiotlb

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.02.19 at 15:10, <michael.d.labriola@xxxxxxxxx> wrote:
> On Wed, Feb 13, 2019 at 5:34 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>
>> >>> On 12.02.19 at 19:46, <michael.d.labriola@xxxxxxxxx> wrote:
>> > Konrad,
>> >
>> > Starting w/ v4.17, I cannot log in to GNOME w/out getting the
>> > following mess in dmesg and ending up back at the GDM login screen.
>> >
>> > [   28.554259] radeon_dp_aux_transfer_native: 200 callbacks suppressed
>> > [   31.219821] radeon 0000:01:00.0: swiotlb buffer is full (sz: 2097152
>> > bytes)
>> > [   31.220030] [drm:radeon_gem_object_create [radeon]] *ERROR* Failed
>> > to allocate GEM object (16384000, 2, 4096, -14)
>> > [   31.226109] radeon 0000:01:00.0: swiotlb buffer is full (sz: 2097152
>> > bytes)
>> > [   31.226300] [drm:radeon_gem_object_create [radeon]] *ERROR* Failed
>> > to allocate GEM object (16384000, 2, 4096, -14)
>> > [   31.300734] gnome-shell[1935]: segfault at 88 ip 00007f39151cd904
>> > sp 00007ffc97611ad8 error 4 in libmutter-cogl.so[7f3915178000+aa000]
>> > [   31.300745] Code: 5f c3 0f 1f 40 00 48 8b 47 78 48 8b 40 40 ff e0
>> > 66 0f 1f 44 00 00 48 8b 47 78 48 8b 40 48 ff e0 66 0f 1f 44 00 00 48
>> > 8b 47 78 <48> 8b 80 88 00 00 00 ff e0 0f 1f 00 48 8b 47 78 48 8b 40 68
>> > ff e0
>> > [   38.193302] radeon_dp_aux_transfer_native: 116 callbacks suppressed
>> > [   40.009317] radeon 0000:01:00.0: swiotlb buffer is full (sz: 2097152
>> > bytes)
>> > [   40.009488] [drm:radeon_gem_object_create [radeon]] *ERROR* Failed
>> > to allocate GEM object (16384000, 2, 4096, -14)
>> > [   40.015114] radeon 0000:01:00.0: swiotlb buffer is full (sz: 2097152
>> > bytes)
>> > [   40.015297] [drm:radeon_gem_object_create [radeon]] *ERROR* Failed
>> > to allocate GEM object (16384000, 2, 4096, -14)
>> > [   40.028302] gnome-shell[2431]: segfault at 2dadf40 ip
>> > 0000000002dadf40 sp 00007ffcd24ea5f8 error 15
>> > [   40.028306] Code: 20 6e 31 00 00 00 00 00 00 00 00 37 e3 3d 2d 7f
>> > 00 00 80 f4 e6 3d 2d 7f 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> > 00 00 00 <00> 00 00 00 00 00 00 00 c1 00 00 00 00 00 00 00 80 e1 d2 03
>> > 00 00
>> >
>> >
>> > This happens w/ both radeon and amdgpu.
>> >
>> > I bisected down to the following range of commits, which basically add
>> > conditional code to radeon and amdgpu to NOT use swiotlb if dma_bits
>> > is smaller than the system's max iomem address...  but that very much
>> > doesn't work on a Xen dom0.
>>
>> Well, not so much a Xen Dom0, but a Xen PV domain.
>>
>> > 82626363 drm: add func to get max iomem address v2
>> > fd5fd480 drm/amdgpu: only enable swiotlb alloc when need v2
>> > 1bc3d3cc drm/radeon: only enable swiotlb path when need v2
>> >
>> > Reverting the offending commits gives me a usable v4.20 dom0 kernel w/
>> > working 3d support.  Not sure what the appropriate upstream fix for
>> > this would be, as I don't 100% understand this.  Could you enlighten
>> > me?  ;-)
>>
>> Well, this depends on how much abstraction we want, and how
>> much abstraction the maintainers of the DRM drivers demand.
>> It could be as simple as adding xen_swiotlb checks into the
>> conditionals setting ->need_swiotlb, but in an abstract sense
>> the issue of course exists for PV guests of any hypervisor.
>> (Altering drm_get_max_iomem() itself would seem wrong to me,
>> unless its name was also changed.)
> 
> Ah, so this isn't necessarily Xen-specific but rather any paravirtual
> guest?  That hadn't crossed my mind.  Is there an easy way to find out
> if we're a pv guest in the need_swiotlb conditionals?

There's xen_pv_domain(), but I think xen_swiotlb would be more to
the point if the check is already to be Xen-specific. There's no generic
"is PV" predicate that I'm aware of.

>  If not, we
> should at least add a module parameter to force swiotlb usage to both
> radeon and amdgpu.  I'd be more than happy to gin up a patch to do
> either and submit to upstream (dri-devel, I guess).

I don't think module parameters are a good way forward here.
They may do as a temporary workaround, but not as a solution.

Jan


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