Re: [PATCH] [v2] linux: virtio: Standardize virtio's concept of "page size"

[Hollis Blanchard <hollisb-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

On Wed, 2008-11-12 at 22:51 +1030, Rusty Russell wrote:
> On Tuesday 11 November 2008 10:07:09 Hollis Blanchard wrote:
> > Both sides of the virtio interface must agree about how big a pfn really
> > is. This is particularly an issue on architectures where the page size is
> > configurable (e.g. PowerPC, IA64) -- the interface must be independent of
> > PAGE_SHIFT.
> >
> > Currently there are three distinct problems:
> > * The shift count used when passing the physical address of the ring to a
> >   PCI-based back end.
> > * The ring layout itself is padded to span at least two "pages".
> > * The balloon driver operates in units of "pages".
> 
> Hi Hollis,
> 
>    The more I thought about this, the more I think we're not solving this
> as neatly as we could.  The trigger was noting that we're breaking the
> userspace API (vring_size and vring_init are exposed to userspace): I
> know that qemu cut & pastes, but that's no excuse.
> 
>    So instead, I've introduced separate constants for each use.  Yes,
> all these constants are 12/4096.  But just to be contrary, at the end
> is a patch to change lguest to 128.  And there's no reason this
> couldn't change in future using some guest detection scheme.

OK. I thought it was simpler to just say "4KB everywhere" in all aspects
of the virtio interface, but I'm happy as long as we solve the problem
somehow. :)

-- 
Hollis Blanchard
IBM Linux Technology Center

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