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

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>]

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
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