Re: [Qemu-devel] [PATCH RFC] vga: flag vga ram for notifiers

[Anthony Liguori]

On 03/31/2011 01:49 PM, Michael S. Tsirkin wrote:
> On Thu, Mar 31, 2011 at 01:33:58PM -0500, Anthony Liguori wrote:
> > On 03/31/2011 12:43 PM, Michael S. Tsirkin wrote:
> > > Currently, vga cards that allocate vga ram,
> > > register it as regular ram. When this happens
> > > a lot, vhost need to get notified and flush
> > > its memory tables, which is slow.
> > >
> > > This was observed with cirrus vga.
> > >
> > > As a solution, add an explicit flag when
> > > registering vga ram, vhost-net can simply ignore it.
> > >
> > > Long term, we might be able to use this API
> > > to avoid the need to request
> > > dirty loggin from devices explicitly.
> > >
> > > Tested: with cirrus vga only.
> > >
> > > Signed-off-by: Michael S. Tsirkin<address@hidden>
> > Treating vga specially is not the right approach.
> >
> > You want to treat real RAM specially and only make that visible to
> > vhost.  See http://wiki.qemu.org/Features/RamAPI
> That seems like a dead project? And VGa is unhandled there.

Just needs some love.

VGA is just another device.  It happens to be that we treat VGA device 
memory as something that behaves like ram occassionally but that does 
not make it RAM.

Something like vhost doesn't need to see anything but RAM.  If we have a 
mechanism to identify RAM as RAM, then vhost can only look at RAM memory 
and not worry about things like VGA.

I thought Alex had gotten a mini-version of RamAPI in but I can't seem 
to figure out what that included.  At any rate, the point is still that 
registering things that you want to exclude in vhost is the wrong 
approach, you want to explicitly mark the things you want to include.

Regards,

Anthony Liguori

> > There is nothing special about VGA.
> It is special in that guest can control host virtual to
> guest physical mappings. In this VGA is similar to
> IO rather than RAM.