Re: large page size virtio issues

[Hollis Blanchard <hollisb@us.ibm.com>]

On Wed, 2008-11-05 at 11:33 -0600, Anthony Liguori wrote:
> Hollis Blanchard wrote:
> > On Wed, 2008-11-05 at 11:25 -0600, Anthony Liguori wrote:
> >   
> >> I think we should just have a VIRTIO_PFN_SHIFT define that is 
> >> architecture specific.
> >>
> >> On x86, it'll be 4k.  You can make it whatever you want for PPC.
> >> Fixing all architectures to be 4k is going to suck for architectures
> >> with < 4k pages.
> >>     
> >
> > So we're now in agreement that it should be hardcoded. I don't see much
> > value in making the actual value arch-specific, but either way it will
> > solve (at least part of, and maybe all of) my problem. Rusty?
> >   
> 
> When you balloon pages, you need to pass something that points to the 
> beginning of a page boundary.  If your page size is 2k, then if you are 
> only able to pass 4k addresses, you can't below any page that happens to 
> not be 4k aligned.
> 
> If you were going to make it common across all architectures, you would 
> have to make it the smallest possible page size and that would break x86 
> backwards compatibility.

OK, makes sense to me.

-- 
Hollis Blanchard
IBM Linux Technology Center