Re: [Xen-devel] [PATCH RFC] drm: add func to better detect wether swiotlb is needed

[Michael Labriola <michael.d.labriola@xxxxxxxxx>]

On Sat, Feb 16, 2019 at 2:00 PM Koenig, Christian
<Christian.Koenig@xxxxxxx> wrote:
>
> Am 15.02.19 um 22:29 schrieb Michael D Labriola:
> > This commit fixes DRM failures on Xen PV systems that were introduced in
> > v4.17 by the following commits:
> >
> > 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
> >
> > The introduction of ->need_swiotlb to the ttm_dma_populate() conditionals
> > in the radeon and amdgpu device drivers causes Gnome to immediately crash
> > on Xen PV systems, returning the user to the login screen.  The following
> > kernel errors get logged:
> >
> > [   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 commit renames drm_get_max_iomem() to drm_need_swiotlb(), adds a
> > xen_pv_domain() check to it, and moves the bit shifting comparison that
> > always follows its usage into the function (simplifying the drm driver
> > code).
>
> You signed of by line is missing. Apart from that it looks like a good
> fix to me.

Oops, I can resend w/ signoff.

> But the question is why does Xen needs dma_alloc_coherent() ?

I'm not sure I understand, even after discussing on xen-devel.  What I
was told by Juergen (on CC) is:

"The thing which is different between Xen PV guests and most others
(all others(?), now that Lguest and UML have been dropped) is that
what Linux thinks of as PFN $N isn't necessarily adjacent to PFN $N+1
in system physical address space.

Therefore, code which has a buffer spanning a page boundary can't just
convert a pointer to the buffer into a physical address, and hand that
address to a device.  You generally end up with either memory
corruption (DMA hitting the wrong page allocated to the guest), or an
IOMMU fault (DMA hitting a pages which isn't allocated to the guest)."

And Christoph (also on CC) pointed out:

"ttm_populate_and_map_pages uses dma_map_page to map GPU memory,
and from what I can tell from your report potentially lots of it.

So this does "properly" use the DMA API for some amount of "properly".
The problem here is that ttm_populate_and_map_pages first allocates
memory and then maps it in a way where it bounce buffers a lot,
leading to a swiotlb buffer exaustion, as seen in your report.

ttm_dma_populate also sort of "properly" uses the DMA API in that it
uses the dma_alloc_coherent allocator.  The benefit of that allocator is
that is always returns addressable memory without exhausing the swiotlb
buffer.  The dowside of ttm_dma_populate / dma_alloc_coherent is that
on architectures where PCIe is not cache coherent it pointlessly
up other resources, as coherent DMA memory can be a very finite resource
there.

So for a short term fix forcing the dma_alloc_coherent route on
Xen/x86 is the right thing.  On Xen/arm and Xen/arm64 is might already
be problemeatic due to the explanation above unfortunately.

The real fix is to finally get broadly available non-coherent device
memory allocator into mainlaine and with ttm_populate_and_map_pages
to use it.  Note that for non-coherent devices it seems like
ttm_populate_and_map_pages also seems to miss proper ownership
management but that is another issue.

My proposal for such an allocator here:

   https://lwn.net/Articles/774429/";

>
> Using dma_alloc_coherent() is really just a fallback path and we need to
> disable a bunch of userspace features when going down that route.

So, for the short-term fix, I'll resubmit my patch w/ appropriate sign-off.

I'm all for making it work even better on Xen after this band-aid is
applied.  I can help test out proposed patches on my systems if that
helps.  Thanks!

-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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