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

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

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


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