Re: [Qemu-devel] [RFC] Memory API

[Anthony Liguori]

On 05/19/2011 08:26 AM, Avi Kivity wrote:
> On 05/19/2011 04:23 PM, Anthony Liguori wrote:
> > > Actually, things are a bit more complicated: This layer has to properly
> > > adopt the coalescing properties of underlying regions or we cause
> > > performance regressions to VGA emulation. That means it has to register
> > > dispatching slots of the corresponding size and set the coalescing flag
> > > accordingly. And it likely need to adjust them as the regions below
> > > change.
> >
> >
> > As I mentioned in another thread, I don't think we want to "design"
> > coalescing into the API. Coalescing is something that breaks through
> > abstractions layers and is really just a hack.
>
> It's impossible not to design it into the API. The layer which wants to
> do coalescing (the device) has no idea if and where its memory is mapped.

There's two places coalescing currently matters: VGA and PCI devices. 
Since VGA is just a special PCI device, let's just focus on PCI devices.

The PCI bus knows exactly where the memory is mapped.  Yes, if you have 
complex IOMMU hierarchies it doesn't, but this is my point.  We don't 
need to design coalesced IO to support these sort of complex hierarchies.



Regards,

Anthony Liguori