Re: [Xen-devel] [Qemu-devel][PATCH] configure: introduce --enable-xen-fb-backend

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

On 04/14/2017 08:52 PM, Stefano Stabellini wrote:
> On Fri, 14 Apr 2017, Juergen Gross wrote:
> > On 14/04/17 08:06, Oleksandr Andrushchenko wrote:
> > > On 04/14/2017 03:12 AM, Stefano Stabellini wrote:
> > > > On Tue, 11 Apr 2017, Oleksandr Andrushchenko wrote:
> > > > > From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> > > > >
> > > > > For some use cases when Xen framebuffer/input backend
> > > > > is not a part of Qemu it is required to disable it,
> > > > > because of conflicting access to input/display devices.
> > > > > Introduce additional configuration option for explicit
> > > > > input/display control.
> > > > In these cases when you don't want xenfb, why don't you just remove
> > > > "vfb" from the xl config file? QEMU only starts the xenfb backend when
> > > > requested by the toolstack.
> > > >
> > > > Is it because you have an alternative xenfb backend? If so, is it really
> > > > fully xenfb compatible, or is it a different protocol? If it is a
> > > > different protocol, I suggest you rename your frontend/backend PV device
> > > > name to something different from "vfb".
> > > Well, offending part is vkbd actually (for multi-touch
> > > we run our own user-space backend which supports
> > > kbd/ptr/mtouch), but vfb and vkbd is the same backend
> > > in QEMU. So, I am ok for vfb, but just want vkbd off
> > > So, there are 2 options:
> > > 1. At compile time remove vkbd and still allow vfb
> > > 2. Remove xenfb completely, if acceptable (this is my case)
> > What about adding a Xenstore entry for backend type and let qemu test
> > for it being not present or containing "qemu"?
sounds reasonable
> That is what we do for the console, using the xenstore node "type". QEMU
> is "ioemu" while xenconsoled is "xenconsoled". Weirdly, instead of a
> backend node, it is a read-only frontend node, see
> tools/libxl/libxl_console.c:libxl__device_console_add.
>
> Oleksandr, I am sorry to feature-creep this simple patch, but I think
> Juergen is right. But we cannot do it just for one protocol. We need to
> introduce a generic way to enable/disable backends in QEMU. Using a
> xenstore node is OK.
agree
> We could do exactly the same as the PV console, thus "type" = "ioemu",
> read-only, under the frontend xenstore directory. Or we could introduce
> new nodes. I would probably go for "backend-type" = "qemu" under the
> backend xenstore directory. I don't have a strong opinion about this. In
> the example below I'll use the PV console convention.
>
> For starters:
>
> * libxl needs to write the "type" node to xenstore for *all* protocols.
>    The "type" is not yet configurable.
> * qemu reads them for all backends, proceeds if "type" = "ioemu"
>
> These should be two simple patches. Stage 2:
>
> * we add options in the xl config file to configure any backend, libxl
>    set "type" accordingly (Maybe not *any*, but vif, vkbd, vfb could all
>    have a "type". It is OK if you only add an option for vkbd.)
> * non-QEMU backends, in particular Linux backends, also read the "type"
>    node and proceed if it's "linux"
>
> Does this sound OK to you?
For the first take it does, but I'll get back to it a bit later
Actually the purpose of the change was to find a way
we can live with backends implemented in QEMU and user-space
and how they can co-exist

Thank you,
Oleksandr

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