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

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH RFC] vga: flag vga ram for notifiers
Date: Thu, 31 Mar 2011 21:07:50 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Thu, Mar 31, 2011 at 02:01:52PM -0500, Anthony Liguori wrote:
> 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.

If we agree on that, will a pair of functions for this work?
How about device_register_ram / device_unregister_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.

Me neither.

>  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

vhost just wants RAM.


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