Re: [Qemu-devel] [PATCH] pci: allow 32-bit PCI IO accesses to pass through the PCI bridge

[Marcel Apfelbaum]

On 23/09/2017 11:23, Mark Cave-Ayland wrote:
> On 22/09/17 23:18, Laszlo Ersek wrote:
>
> > On 09/22/17 14:18, Mark Cave-Ayland wrote:
> > > Whilst the underlying PCI bridge implementation supports 32-bit PCI IO
> > > accesses, unfortunately they are truncated at the legacy 64K limit.
> > >
> > > Signed-off-by: Mark Cave-Ayland <address@hidden>
> > > ---
> > >   hw/pci/pci_bridge.c |    3 ++-
> > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/hw/pci/pci_bridge.c b/hw/pci/pci_bridge.c
> > > index 17feae5..a47d257 100644
> > > --- a/hw/pci/pci_bridge.c
> > > +++ b/hw/pci/pci_bridge.c
> > > @@ -379,7 +379,8 @@ void pci_bridge_initfn(PCIDevice *dev, const char *typename)
> > >       sec_bus->address_space_mem = &br->address_space_mem;
> > >       memory_region_init(&br->address_space_mem, OBJECT(br), "pci_bridge_pci", 
> > > UINT64_MAX);
> > >       sec_bus->address_space_io = &br->address_space_io;
> > > -    memory_region_init(&br->address_space_io, OBJECT(br), "pci_bridge_io", 
> > > 65536);
> > > +    memory_region_init(&br->address_space_io, OBJECT(br), "pci_bridge_io",
> > > +                       UINT32_MAX);
> > >       br->windows = pci_bridge_region_init(br);
> > >       QLIST_INIT(&sec_bus->child);
> > >       QLIST_INSERT_HEAD(&parent->child, sec_bus, sibling);

Hi Mark,

> > Based on the commit message, I assume this change is guest-visible. If
> > so, should it be made dependent on a compat property, so that it doesn't
> > cause problems with migration?
>
> In order to enable 32-bit IO accesses the PCI bridge needs to set bit 0
> in the IO_LIMIT and IO_BASE registers - this bit is read-only to guests,
> so unless a PCI bridge has this bit set then it's impossible for this
> change to be guest visible.
>
> I did a grep for PCI_IO_RANGE_TYPE_32 and didn't see any existing users
> (other than an upcoming patchset from me!), so this combined with the
> fact that without this patch the feature is broken makes me think that I
> am the first user and so existing guests won't have a problem.

(adding Dave for his expertise)

Do you know how the migration code will behave if it will have
a 65k address space on source and MAX UINT on destination?
(and the other way around for rolling back)

Thanks,
Marcel

> ATB,
>
> Mark.