Re: Upstream Dom0 DRM problems regarding swiotlb

[Michael Labriola <michael.d.labriola@gmail.com>]

On Wed, Feb 13, 2019 at 9:28 AM Jan Beulich <JBeulich@suse.com> wrote:
>
> >>> On 13.02.19 at 15:10, <michael.d.labriola@gmail.com> wrote:
> > On Wed, Feb 13, 2019 at 5:34 AM Jan Beulich <JBeulich@suse.com> wrote:
> >>
> >> >>> On 12.02.19 at 19:46, <michael.d.labriola@gmail.com> 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.)

So, the commit message for the patch that added drm_get_max_iomem()
specifically states that the function "will be used to check if the
driver needs swiotlb"...  Sounds like a logical place to put some type
of PV conditional to me.  I get that that seems a bit sneaky and
unrelated to the function's name... but it definitely plays directly
into the function's stated purpose.

> >
> > 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.

Well, that makes doing conditional code right more difficult.  I
assume since there isn't a generic predicate, and PV isn't new, that
it's absence is by design?  To reign in the temptation to sprinkle
conditional code all over the kernel?  ;-)

>
> >  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.

Agreed.  I suppose not many people have bumped into this problem in
the wild, since it's been in mainline since 4.17.  Am I really the
only person running a development system in Xen w/ AMD video cards who
expects 3d to work?  Didn't really seem like a strange workload to
me... especially with so many desktop environments requiring GL
support nowadays.

-Mike

-- 
Michael D Labriola
21 Rip Van Winkle Cir
Warwick, RI 02886
401-316-9844 (cell)
401-848-8871 (work)
401-234-1306 (home)

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