Re: [PATCH v3 2/6] libxl: do not add a vkb backend to hvm guests

[Ian Campbell <ian.campbell@citrix.com>]

On Tue, 2015-06-30 at 15:02 +0100, Stefano Stabellini wrote:
> On Tue, 30 Jun 2015, Ian Campbell wrote:
> > On Tue, 2015-06-30 at 12:21 +0100, Stefano Stabellini wrote:
> > > On Tue, 30 Jun 2015, Ian Campbell wrote:
> > > > On Mon, 2015-06-29 at 18:59 +0100, Stefano Stabellini wrote:
> > > > > On Thu, 25 Jun 2015, Ian Campbell wrote:
> > > > > > On Tue, 2015-06-16 at 16:39 +0100, Stefano Stabellini wrote:
> > > > > > > On Tue, 16 Jun 2015, Wei Liu wrote:
> > > > > > > > On Wed, Jun 10, 2015 at 11:09:50AM +0100, Stefano Stabellini wrote:
> > > > > > > > > When QEMU restricts its xenstore connection, it cannot provide PV
> > > > > > > > > backends. A separate QEMU instance is required to provide PV backends in
> > > > > > > > > userspace, such as qdisk. With two separate instances, it is not
> > > > > > > > > possible to take advantage of vkb for mouse and keyboard, as the QEMU
> > > > > > > > > that emulates the graphic card (the device model), would be separate
> > > > > > > > > from the QEMU running the vkb backend (PV QEMU).
> > > > > > > > > 
> > > > > > > > 
> > > > > > > > The question is that how would this affect the non-split setup.
> > > > > > > 
> > > > > > > vkb is useful because emulating usb forces QEMU to wake up more often.
> > > > > > > However there is no way around it.
> > > > > > 
> > > > > > Does pvfb+vkb continue to work due to code somewhere else?
> > > > > 
> > > > > Yes, it continues to work as usual for PV guests.
> > > > > 
> > > > > 
> > > > > > Do we think anyone will actually be using emulated VGA + PV input
> > > > > > devices?
> > > > > 
> > > > > VGA + PV input only works with Linux and is only useful for power
> > > > > efficiency, because if you disable usb emulation in QEMU, then QEMU
> > > > > would be able to wake up less often. Given that usb emulation is still
> > > > > on by default, I don't think that this change will have a big impact.
> > > > 
> > > > My question was whether we thought anyone would be using this
> > > > non-default configuration, not what the impact on the default is.
> > > > 
> > > > You gave a good reason why people might be using this facility, do you
> > > > think anyone is actually using it?
> > >  
> > > I don't know of anybody using it. I don't think we made clear enough how
> > > to use this non-default configuration and its advantages for users to go
> > > out of their ways to use it. 
> > 
> > That's good enough for me, thanks,.
> 
> Can I add your acked-by?

If you put some distillation of the reasoning given in this subthread
for why we think we can get away with it into the commit message then
yes.

Ian.