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

[Oleksandr Andrushchenko <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>>:
> > 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!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
>
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