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

[Anthony Liguori]

On 03/31/2011 04:37 PM, Michael S. Tsirkin wrote:
> 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
>
> BTW, what is IO_MEM_NOTDIRTY?

TCG needs to keep track of dirty memory in order to be able to deal with 
self modifying code.  IO_MEM_NOTDIRTY is the state that RAM takes before 
the first write.  I don't think IO_MEM_NOTDIRTY would ever show up in 
the l1_phys_map but it will show up in the iotlb.  The iotlb uses the 
same dispatch code as the l1_phys_map though.

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.