Re: [PATCH RFC] virtio-pci: new config layout: using memory BAR

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

On Wed, Jun 05, 2013 at 03:45:09PM -0500, Anthony Liguori wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > On Wed, Jun 05, 2013 at 10:43:17PM +0300, Michael S. Tsirkin wrote:
> >> On Wed, Jun 05, 2013 at 01:57:16PM -0500, Anthony Liguori wrote:
> >> > "Michael S. Tsirkin" <mst@redhat.com> writes:
> >> > 
> >> > > On Wed, Jun 05, 2013 at 10:46:15AM -0500, Anthony Liguori wrote:
> >> > >> Look, it's very simple.
> >> > > We only need to do it if we do a change that breaks guests.
> >> > >
> >> > > Please find a guest that is broken by the patches. You won't find any.
> >> > 
> >> > I think the problem in this whole discussion is that we're talking past
> >> > each other.
> >> > 
> >> > Here is my understanding:
> >> > 
> >> > 1) PCI-e says that you must be able to disable IO bars and still have a
> >> > functioning device.
> >> > 
> >> > 2) It says (1) because you must size IO bars to 4096 which means that
> >> > practically speaking, once you enable a dozen or so PIO bars, you run
> >> > out of PIO space (16 * 4k == 64k and not all that space can be used).
> >> 
> >> 
> >> Let me add 3 other issues which I mentioned and you seem to miss:
> >> 
> >> 3) architectures which don't have fast access to IO ports, exist
> >>    virtio does not work there ATM
> >> 
> >> 4) setups with many PCI bridges exist and have the same issue
> >>    as PCI express. virtio does not work there ATM
> >> 
> >> 5) On x86, even with nested page tables, firmware only decodes
> >>    the page address on an invalid PTE, not the data. You need to
> >>    emulate the guest to get at the data. Without
> >>    nested page tables, we have to do page table walk and emulate
> >>    to get both address and data. Since this is how MMIO
> >>    is implemented in kvm on x86, MMIO is much slower than PIO
> >>    (with nested page tables by a factor of >2, did not test without).
> >
> > Oh I forgot:
> >
> > 6) access to MMIO BARs is painful in the BIOS environment
> >    so BIOS would typically need to enable IO for the boot device.
> 
> But if you want to boot from the 16th device, the BIOS needs to solve
> this problem anyway.
> 
> Regards,
> 
> Anthony Liguori

But it's solvable: just enable devices one by one to boot from them.

-- 
MST