linux-kernel - Re: [PATCH v7] Add udmabuf misc device

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Date:   Mon, 10 Sep 2018 15:30:50 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Gert Wollny <gert.wollny@...labora.com>
Cc:     linux-kernel@...r.kernel.org, David Airlie <airlied@...ux.ie>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v7] Add udmabuf misc device

On Mon, Sep 10, 2018 at 01:31:08PM +0200, Gert Wollny wrote:
> Am Montag, den 10.09.2018, 12:53 +0200 schrieb Gerd Hoffmann:
> > 
> > By default qemu doesn't use memfd for backing storage, you have to
> > explicitly configure qemu that way (see qemu commit log of the test
> > branch):
> > 
> >     qemu-system-x86_64 -m 2G
> >             -object memory-backend-memfd,id=ram,size=2G
> >             -numa node,memdev=ram"
> 
> Thanks, but that doesn't seem to help.

Note the memory is specified twice, once with -m, once for the
memory-backend-memfd object.  The two must match of course (or, to be
exact, the sum of all memory backends must match the total memory, but
unless you want create a virtual numa machine there is no reason to have
multiple memory-backends).

> Are there any host kernel
> configuration parameters or features that I should set (apart from
> enabling udmabuf)? 

I'm not aware of any unusual parameters required.  My config is based on
rhel-7, then stripped down with localmodconfig, then enabled the bits I
want in my test kernel.

Oh, wait, CONFIG_MEMFD_CREATE could be the one.  I guess I should add a
Kconfig dependency.  Even though udmabuf will build just fine without
that (because memfd is basically shmem with some features on top) it
doesn't buy you much if the memfd_create syscall is not available ...

cheers,
  Gerd