Re: [kvm-ppc-devel] booting from virtio-blk

Hollis Blanchard
2008-04-01T21:18:17Z

On Tue, 2008-04-01 at 16:03 -0500, Anthony Liguori wrote:
> Benjamin Herrenschmidt wrote:
> > On Tue, 2008-04-01 at 12:09 -0500, Anthony Liguori wrote:
> >
> >
> >> It's the unfortunate side-effect of using PCI config space without
> >> passing it's semantics through to the virtio devices. Right now, you do
> >> a config_get which is basically a memcpy. If we didn't do accesses with
> >> ioread8(), you could potentially have a caller than did a config_get()
> >> of size 4 that didn't intend on having endian conversion applied.
> >>
> >> The other option would have been to provide config_get() and
> >> config_get8/16/32/64() the later performing endian conversion.
> >>
> >
> > Config space should be 8/16/32. Is that ever bridged to real PCI config
> > space anyway ? Or only virtio ? And it should be endian swapped at the
> > low level, either by your HV calls or by the low level kernel. Always.
> > That's how PCI config space is supposed to work.

Virtio accesses will not be bridged to real PCI space.

> I guess the point is, is that virtio config space is an abstraction with
> the implementation that is based on PCI converting all accesses to a
> series of 8-bit accesses. The virtio config space happens to be little
> endian just like the PCI config space.

The point is that a virtio device appears as a PCI device. Like all
other PCI devices, it has config space. Unlike all other PCI devices,
its config space is accessed with 1-byte reads.

--
Hollis Blanchard
IBM Linux Technology Center