[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

  • To: Michael Labriola <michael.d.labriola@xxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Thu, 14 Feb 2019 00:11:02 +0000
  • Autocrypt: addr=andrew.cooper3@xxxxxxxxxx; prefer-encrypt=mutual; keydata= mQINBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABtClBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPokCOgQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86LkCDQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAYkC HwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA==
  • Cc: Paul Durrant <Paul.Durrant@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxx, Jan Beulich <JBeulich@xxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
  • Delivery-date: Thu, 14 Feb 2019 00:11:14 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 13/02/2019 21:08, Michael Labriola wrote:
> On Wed, Feb 13, 2019 at 3:21 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> wrote:
>> On 13/02/2019 20:15, Michael Labriola wrote:
>>> On Wed, Feb 13, 2019 at 2:16 PM Konrad Rzeszutek Wilk
>>> <konrad.wilk@xxxxxxxxxx> wrote:
>>>> On Wed, Feb 13, 2019 at 01:38:21PM -0500, Michael Labriola wrote:
>>>>> On Wed, Feb 13, 2019 at 1:16 PM Michael Labriola
>>>>> <michael.d.labriola@xxxxxxxxx> wrote:
>>>>>> On Wed, Feb 13, 2019 at 11:57 AM Konrad Rzeszutek Wilk
>>>>>> <konrad.wilk@xxxxxxxxxx> wrote:
>>>>>>> On Wed, Feb 13, 2019 at 09:09:32AM -0700, Jan Beulich wrote:
>>>>>>>>>>> On 13.02.19 at 17:00, <michael.d.labriola@xxxxxxxxx> wrote:
>>>>>>>>> On Wed, Feb 13, 2019 at 9:28 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>>>>>>>>>> On 13.02.19 at 15:10, <michael.d.labriola@xxxxxxxxx> wrote:
>>>>>>>>>>> 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?  ;-)
>>>>>>>> Well, with lguest gone, Xen is the only PV environment the kernel
>>>>>>>> can run in, afaik at least. I guess to decide between the suggested
>>>>>>>> options or the need for some abstracting macro (or yet something
>>>>>>>> else), you'll really need to ask the driver maintainers. Or simply
>>>>>>>> send a patch their way implementing one of them, and see what
>>>>>>>> their reaction is.
>>>>>>> Could you try this out and see if it works and I will send it out:
>>>>> *snip*
>>>>>> Yes, that works for me.  However, I feel like the conditional should
>>>>>> be in drm_get_max_iomem() instead of directly after it everywhere it's
>>>>>> used...  and is just checking xen_pv_domain() enough?  Jan made it
>>>>>> sound like there were possibly other PV cases that would also still
>>>>>> need swiotlb.
>>>>> How about this?  It strcmp's pv_info to see if we're bare metal, does
>>>>> the comparison in a single place, moves the bit shifting comparison
>>>>> into the function (simplifying the drm driver code), and renames the
>>>>> function to more aptly describe what's going on.
>>>> <nods> That looks much better.
>>> Great!  Now the only question left is:  KVM, VMware, Xen PVH, Xen HVM,
>>> and Xen PV all populate pv_info.  Do any of those other than Xen PV
>>> *really* need swiotlb.  That's slightly over my head.  As written, my
>>> patch would require swiotlb for all of them because I was attempting
>>> to not be Xen-specific.
>> Its far more complicated that "Xen PV requires swiotlb".
>> I presume the underlying problem here is DRM being special and not
>> DMA-mapping its buffers, and trying to DMA to a buffer crossing a 4k
>> boundary?
> Well, I don't 100% understand how all these things work...  but here's
> what I do know.  There are a series of commits in v4.17 that try to
> optimize the radeon and amdgpu drivers by skipping calls to
> ttm_dma_populate() and ttm_dma_unpopulate() unless they're "really
> needed".  The original commit determines if swiotlb is needed by
> checking to see if the max io mapping address of system memory is over
> the video card's accessing range.  I can no longer log into Gnome on a
> Xen dom0 after upgrading my kernel to v4.20 because the code that's no
> longer happening was actually needed in a paravirtualized environment.

But from the log you provided, your bug was space exhaustion in the
swiotlb, no?

> So, I'm trying to get all my details straight so I can submit a patch
> to fix it w/out saying anything factually incorrect.

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).

Xen PV is very good at finding DMA bugs in drivers.  The way to resolve
this is to fix the driver to use the proper DMA APIs - not to add even
more magic corner cases.

In general, a lot of devices can do 4k scatter/gather, or end up making
requests to buffers which fit within a single page, but the SWIOTLB does
act as a mechanism of last resort.  It has a massive performance penalty
(due to double buffering), and does have a tendency to fragment (due to
asymmetric size requests).

However, there is one DMA mode (in the process of getting properly
upstream, but has been used for several years by various downstreams)
where IOVA == Linux's idea of contiguous PFN space, so you can do odd
sized DMAs which cross page boundaries.

The point is that the DMA ops (and *only* the DMA ops, from a
correctness standpoint) know how to convert PFNs into IO-virtual
addresses for devices, because it may not be a 1:1 mapping.  Nothing
else in the kernel can legitimately be making decisions like this.


Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.