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

[Rusty Russell <rusty@rustcorp.com.au>]

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.

Patch stream to follow...
Rusty.