Re: [PATCH v3 00/16] virtio-pci: towards virtio 1.0 guest support

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jan 16, 2015 at 09:32:26AM +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> > >  * I'd like to see some more flexibility in the pci bar layout.  Stuff
> > >    I have in mind:
> > >     - New devices which don't need a legacy bar can use bar 0 for
> > >       modern.
> > >     - One MMIO bar is enough, we can place both virtio regions and
> > >       msi-x regions there.  I'd suggest to add msi-x sub-regions to
> > >       the modern bar.
> > 
> > Why exactly? It seems simpler to separate things, extra BARs
> > have no cost.
> 
> Well, there are only six BARs.  legacy bar, modern mmio bar, msi-x bar,
> modern io bar (for fast isr).  That already four out of six ...

So?  If we need to add another bar, we'll change things.

> We have a mmio bar, which we partition into subregions for virtio-1.0
> anyway.  Also placing msi-x there is a single msix_init() call.  xhci is
> doing that too:
> 
>     00000000febf0000-00000000febf3fff (prio 1, RW): xhci
>       00000000febf0000-00000000febf003f (prio 0, RW): capabilities
>       00000000febf0040-00000000febf043f (prio 0, RW): operational
>       00000000febf0440-00000000febf044f (prio 0, RW): usb3 port #1
>       00000000febf0450-00000000febf045f (prio 0, RW): usb3 port #2
>       00000000febf0460-00000000febf046f (prio 0, RW): usb3 port #3
>       00000000febf0470-00000000febf047f (prio 0, RW): usb3 port #4
>       00000000febf0480-00000000febf048f (prio 0, RW): usb2 port #1
>       00000000febf0490-00000000febf049f (prio 0, RW): usb2 port #2
>       00000000febf04a0-00000000febf04af (prio 0, RW): usb2 port #3
>       00000000febf04b0-00000000febf04bf (prio 0, RW): usb2 port #4
>       00000000febf1000-00000000febf121f (prio 0, RW): runtime
>       00000000febf2000-00000000febf281f (prio 0, RW): doorbell
>       00000000febf3000-00000000febf30ff (prio 0, RW): msix-table
>       00000000febf3800-00000000febf3807 (prio 0, RW): msix-pba
> 

However that one is non prefetheable, and I prefer putting
virtio caps in a prefetcheable bar.

> > >  * What is the reason for making the modern bar 8M in size?  Looks a bit
> > >    excessive, given that only 64k or so of that are actually used ...
> > 
> > I use a page per VQ for architectures that can locate the offset of the
> > accessed page that triggered EPT violation faster than the offset within
> > page.  I think this is the case for SVM.
> 
> 8M still looks excessive, given that we typically have a small number of
> queues per device.  Do you allocate address space for the maximum
> possible number of queues unconditionally?

Yes, simpler this way. I'll check if I can find the real # of VQs.

> > > virtio-scsi seems to be broken, at least my usual fedora guest didn't
> > > boot up from virtio-scsi disk when using a guest kernel with this patch
> > > series applied.
> > 
> > I'll re-test. Do other devices work for you? Thanks!
> 
> Didn't came very far yet in my testing due to the guest not booting.

Try virtio-blk - that worked for me.

>  I
> plan to try other storage for the image, but didn't found the time yet.
> 
> I've tried to boot a F21 live iso with virtio-net (legacy guest driver
> obviously), which seems to work ok in light testing.
> 
> BTW: is there a tool (or pciutils patch) which can decode the virtio
> capabilities?
> 
> cheers,
>   Gerd
>