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

[Marcel Apfelbaum <marcel@redhat.com>]

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 <mark.cave-ayland@ilande.co.uk>
>>> ---
>>>   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.
>