Xen project Mailing List

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.

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>>:
> 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!mz3gn1wQMX2DXeNuAV-1_dI7nxFYYZOgdPiJNSFMesCz9lAzOKlwVPlddbxbcLmUO44NOy0TFA$>
>
> Best regards,
> Alexander Sychev

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