Re: [Xen-devel] PV DRM doesn't work without auto_translated_physmap feature in Dom0

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>]

On 2/5/20 8:59 PM, Santucco wrote:
> Hello,
> Ok, I  commented out the memcpy call and run the test.
> displ_be hasn’t crached, I have seen FLIP events in the log.
> But there hasn’t been the black screen, just a blink effect every 
> couple of seconds.
> Logs are attached.
Ok, so I believe that frontend - backend (displ_be) communication is ok
and there is nothing to do there.

Next, I would start debugging the following in Xen:
(XEN) mm.c:2223:d2v0 Bad L1 flags 80
and have a look at [1]. Probably, someone on Xen x86 side can tell
if this could be related to the flags at [2].

> Best regards,
> Alexander
>
>     Среда, 5 февраля 2020, 9:31 +03:00 от Oleksandr Andrushchenko
>     <oleksandr_andrushchenko@xxxxxxxx>:
>     On 2/4/20 10:28 AM, Santucco wrote:
>     > Hello,
>     > displ_be was compiled without zero-copy support early.
>     > I have tried with the recompiled dom0 kernel, a result is the same.
>     > Logs and configs (+displ_be’s CMakeCache.txt ) are attached.
>     Ok, yet another test to localize the problem.
>     Could you please remove memcpy from
>     #1  0x000055e5a1f28bec in Drm::DumbDrm::copy (this=0x7f9338000e00) at
>     /home/santucco/tmp/xen-troops/displ_be/src/displayBackend/drm/Dumb.cpp:149
>     and just memset the destination with 0 or whatever.
>
>     I expect that system won't crash, nothing will be shown (black
>     screen), but
>     displ_be will show page flip events in its logs.
>     > Best regards,
>     > Alexander
>     >
>     > Понедельник, 3 февраля 2020, 10:36 +03:00 от Oleksandr
>     > Andrushchenko <oleksandr_andrushchenko@xxxxxxxx
>     </compose?To=oleksandr_andrushchenko@xxxxxxxx>>:
>     >
>     >
>     > On 2/1/20 4:39 PM, Santucco wrote:
>     > > Hello again,
>     > > I have not yet made to work my drm client, so I have tried to run
>     > > linux like a domU (to see how it should work), it doesn’t work too
>     > > — displ_be catches SIGSEGV:
>     > >
>     > > #0  0x00007f4afed1c161 in ?? () from /lib64/libc.so.6
>     > > #1  0x000055723b9c5bec in Drm::DumbDrm::copy
>     > (this=0x7f4adc000e00) at
>     > >
>     >
>     /home/santucco/tmp/xen-troops/displ_be/src/displayBackend/drm/Dumb.cpp:149
>     > > #2  0x000055723b9a8f51 in BuffersStorage::getFrameBufferAndCopy
>     > > (this=0x7f4ae00010e0, fbCookie=18446612682295083264) at
>     > >
>     >
>     
> /home/santucco/tmp/xen-troops/displ_be/src/displayBackend/BuffersStorage.cpp:165
>     > > It tries to copy to mBuffer with non-accessible address.
>     > > For the moment I see a strange offset for mmap call of
>     > /dev/drm/card0
>     > > in the strace log — 0x100000000. Is that normal?
>     > > Any direction of which to dig will be very helpful.
>     > > Configuration details:
>     > > Xen 4.12.1 Dom0: Linux 4.20.17-gentoo #13 SMP Sat Dec 28
>     > 11:12:24 MSK
>     > > 2019 x86_64 Intel(R) Celeron(R) CPU N3050 @ 1.60GHz GenuineIntel
>     > GNU/Linux
>     > > DomU: Linux 4.20.17-gentoo
>     > > last xen-troops/libxenbe and xen-troops/displ_be
>     > > Logs (dmesg, xl dmesg, displ_be, strace log of displ_be), a
>     > backtrace
>     > > of gdb and kernel configs are attached.
>     > > Thanks in advance.
>     > Could you please try Dom0 kernel WITHOUT the options below:
>     > CONFIG_XEN_GNTDEV_DMABUF=y
>     > CONFIG_XEN_GRANT_DMA_ALLOC=y
>     >
>     > Then, just to make sure, did you build displ_be without zero-copy
>     > support?
>     >
>     > > On 1/8/20 5:38 PM, Santucco wrote:
>     > > > Thank you very much for all your answers.
>     > > >
>     > > > Среда, 8 января 2020, 10:54 +03:00 от Oleksandr Andrushchenko
>     > > > <oleksandr_andrushchenko@xxxxxxxx
>     </compose?To=oleksandr_andrushchenko@xxxxxxxx>
>     > > > </compose?To=oleksandr_andrushchenko@xxxxxxxx>>:
>     > > > On 1/6/20 10:38 AM, Jürgen Groß wrote:
>     > > > > On 06.01.20 08:56, Santucco wrote:
>     > > > >> Hello,
>     > > > >>
>     > > > >> I’m trying to use vdispl interface from PV OS, it doesn’t
>     work.
>     > > > >> Configuration details:
>     > > > >>      Xen 4.12.1
>     > > > >>      Dom0: Linux 4.20.17-gentoo #13 SMP Sat Dec 28
>     11:12:24 MSK
>     > > > 2019
>     > > > >> x86_64 Intel(R) Celeron(R) CPU N3050 @ 1.60GHz GenuineIntel
>     > > > GNU/Linux
>     > > > >>      DomU: x86 Plan9, PV
>     > > > >> displ_be as a backend for vdispl and vkb
>     > > > >>
>     > > > >> when VM starts, displ_be reports about an error:
>     > > > >> gnttab: error: ioctl DMABUF_EXP_FROM_REFS failed: Invalid
>     > argument
>     > > > >> (displ_be.log:221)
>     > > > >>
>     > > > >> related Dom0 output is:
>     > > > >> [ 191.579278] Cannot provide dma-buf: use_ptemode 1
>     > > > >> (dmesg.create.log:123)
>     > > > >
>     > > > > This seems to be a limitation of the xen dma-buf driver.
>     It was
>     > > > written
>     > > > > for being used on ARM initially where PV is not available.
>     > > > This is true and we never tried/targeted PV domains with this
>     > > > implementation,
>     > > > so if there is a need for that someone has to take a look on the
>     > > > proper
>     > > > implementation for PV…
>     > > >
>     > > > Have I got your right and there is no the proper
>     > implementation :-)?
>     > > There is no
>     > > >
>     > > > >
>     > > > > CC-ing Oleksandr Andrushchenko who is the author of that
>     > driver. He
>     > > > > should be able to tell us what would be needed to enable PV
>     > dom0.
>     > > > >
>     > > > > Depending on your use case it might be possible to use PVH
>     > dom0, but
>     > > > > support for this mode is "experimental" only and some features
>     > > > are not
>     > > > > yet working.
>     > > > >
>     > > > Well, one of the workarounds possible is to drop zero-copying
>     > use-case
>     > > > (this is why display backend tries to create dmu-bufs from
>     grants
>     > > > passed
>     > > > by the guest domain and fails because of "Cannot provide
>     dma-buf:
>     > > > use_ptemode 1")
>     > > > So, in this case display backend will do memory copying for the
>     > > > incoming
>     > > > frames
>     > > > and won't touch DMABUF_EXP_FROM_REFS ioctl.
>     > > > To do so just disable zero-copying while building the
>     backend [1]
>     > > >
>     > > > Thanks, I have just tried the workaround. The backend has failed
>     > > > in an other place not corresponding with dma_buf.
>     > > > Anyway it is enough to continue debugging  my
>     > frontend implementation.
>     > > > Do you know how big is performance penalty in comparison with
>     > > > the zero-copy variant?
>     > > Well, it solely depends on your setup, so I cannot tell what
>     > > would the numbers be in your case. Comparing to what I have
>     doesn't
>     > > make any sense to me: one should compare apples to apples
>     > > > Does it make a sense if I make a dedicated HVM domain with
>     > linux only
>     > > > for the purpose of vdispl and vkbd backends? Is there a
>     hope this
>     > > > approach will work?
>     > > You can try if this approach fits your design and requirements
>     > > >
>     > > > >
>     > > > > Juergen
>     > > > >
>     > > > [1]
>     > > >
>     > >
>     >
>     https://github.com/xen-troops/displ_be/blob/master/CMakeLists.txt#L12
>     
> <https://urldefense.com/v3/__https://github.com/xen-troops/displ_be/blob/master/CMakeLists.txt*L12__;Iw!!GF_29dbcQIUBPA!kZ1JQFRS2pXj_IuXBhvYhmP9Q_svcLyjCXK9465ULGB4MeiYPRz2cF7lepHggr9UxPU9zOBEUw$>
>     >
>     
> <https://urldefense.com/v3/__https://github.com/xen-troops/displ_be/blob/master/CMakeLists.txt*L12__;Iw!!GF_29dbcQIUBPA!kvDgy3X0IuSQk7D2DdsGtsjtyGroYbNKOrPG95OpyoAkuBVbFSmzozwfor05jkRl0ita0FumBw$>
>     > >
>     >
>     
> <https://urldefense.com/v3/__https://github.com/xen-troops/displ_be/blob/master/CMakeLists.txt*L12__;Iw!!GF_29dbcQIUBPA!gi81oZZNvWaFWUVnaZluA_mNBAItLMd4RZmnc-M_FmlpDojqeQQnS7aXSNlbo80re9uOl2wqFA$>
>     > > >
>     > >
>     >
>     
> <https://urldefense.com/v3/__https://github.com/xen-troops/displ_be/blob/master/CMakeLists.txt*L12__;Iw!!GF_29dbcQIUBPA!mz3gn1wQMX2DXeNuAV-1_dI7nxFYYZOgdPiJNSFMesCz9lAzOKlwVPlddbxbcLmUO44NOy0TFA$>
>     > > >
>     > > > Best regards,
>     > > >   Alexander Sychev
>     > >
>     >
>     >
>     ------------------------------------------------------------------------
>
[1] https://elixir.bootlin.com/linux/v5.5/source/drivers/xen/gntdev.c#L300
[2] https://elixir.bootlin.com/linux/v5.5/source/drivers/xen/gntdev.c#L319
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