Re: large page size virtio issues

[Anthony Liguori <aliguori@us.ibm.com>]

Hollis Blanchard wrote:
> Actually there's an additional complication: PAGE_SIZE is entrenched in
> the layout of the ring structure itself (large "struct vring" comment in
> include/linux/virtio_ring.h). Callers of vring_size() demonstrate this
> problem.
>
> I believe the vring is split across two pages so that it could be
> directly mapped by two untrusted guests: the producer would have RW
> access to the descriptors and the "available" fields, while the consumer
> would have only RO access. I don't think this is yet implemented; is it
> still planned?
>   

Rusty experimented with it but I don't think anyone has done anything 
seriously with it.

> It looks like vring_size() et al were carefully written to allow
> arbitrary page sizes, so for now I'll assume that a struct vring that is
> contained completely within a single guest mapping is OK and work up a
> patch.
>   

It was written that way so that vring_size() could be used in userspace 
where there isn't a PAGE_SIZE defined.  Rusty just passes in 
getpagesize() in the lguest userspace.

Pure evil if you ask me :-)

> It's worth noting that the PFN_SHIFT question (in the other thread) is a
> separate issue. I could have PFN_SHIFT=10 but define VRING_PAGE_SIZE=4K
> for internal alignment. I'm a little nervous about making PPC the only
> arch with VRING_PAGE_SIZE=1K, since that might affect my vring layout in
> "unusual" ways if it expands beyond 1K but stays inside 4K.
>   

This is starting to get a little hairy.  Maybe 64k guest pages is asking 
for trouble?  Are you sure other things aren't breaking all over the 
place with these patches in place?

Regards,

Anthony Liguori