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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 23:37:43 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Thu, Mar 31, 2011 at 04:32:44PM -0500, Anthony Liguori wrote:
> On 03/31/2011 04:26 PM, Michael S. Tsirkin wrote:
> >On Thu, Mar 31, 2011 at 02:29:50PM -0500, Anthony Liguori wrote:
> >>On 03/31/2011 02:18 PM, Peter Maydell wrote:
> >>>On 31 March 2011 20:01, Anthony Liguori<address@hidden>   wrote:
> >>>>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.
> >>>So, to ask a dumb question, what does make something RAM?
> >>It's a made up concept that we use to make device performance faster.
> >>
> >>Basically, RAM should include all of the memory that a reasonable
> >>device (that we control) would DMA to and has a relatively stable
> >>mapping.
> >>
> >>>My take on RAM is that RAM is just another device; the only
> >>>difference is that you want to be able to implement fast
> >>>paths that go straight(ish) to target memory; but that's
> >>>an optimisation detail, not something that makes RAM
> >>>conceptually different from other devices...
> >>Right, the trouble is, if you want to treat RAM like any other
> >>device, you can't get stable mappings to it which is bad for
> >>something like vhost-net.
> >>
> >>Regards,
> >>
> >>Anthony Liguori
> >Not only that I guess. Removing the VGA memory with the baloon
> >will likely also be a bad idea.
> It's just the equivalent of a memset(0).  It would be a silly thing
> for a guest to do but not somethign to be concerned about.
> Regards,
> Anthony Liguori

I thought another way would be to replace IO_MEM_RAM with
IO_MEM_DEVICE_SHADOW_RAM in these cases but no idea
what the right value for that enum would be.


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