Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed

[Avi Kivity]

Anthony Liguori wrote:
> Paul Brook wrote:
> > On Friday 03 October 2008, Ryan Harper wrote:
> >  
> > > The default buffer size breaks up larger read/write requests 
> > > unnecessarily.
> > > When we encounter requests larger than the default dma buffer, 
> > > reallocate
> > > the buffer to support the request.
> > >     
> >
> > Allocating unboundedly large host buffers based on guest input seems 
> > like a bad idea.
> >   
>
> Perhaps they could be at least bound to phys_ram_size.

So the guest could double the memory load on the host with just one 
request?!

> In general, I don't think there's a correct size to bound them that's 
> less than phys_ram_size.  The guest may be issuing really big IO 
> requests.

The correct fix is not to buffer at all but use scatter-gather.  Until 
this is done buffering has to be bounded.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.