Re: [Qemu-devel] Using PCI config space to indicate config location

[Rusty Russell <rusty@rustcorp.com.au>]

Anthony Liguori <aliguori@us.ibm.com> writes:
> Gerd Hoffmann <kraxel@redhat.com> writes:
>
>>   Hi,
>>
>>>> So we could have for virtio something like this:
>>>>
>>>>         Capabilities: [??] virtio-regs:
>>>>                 legacy: BAR=0 offset=0
>>>>                 virtio-pci: BAR=1 offset=1000
>>>>                 virtio-cfg: BAR=1 offset=1800
>>> 
>>> This would be a vendor specific PCI capability so lspci wouldn't
>>> automatically know how to parse it.
>>
>> Sure, would need a patch to actually parse+print the cap,
>> /me was just trying to make my point clear in a simple way.
>>
>>>>>> 2) ISTR an argument about mapping the ISR register separately, for
>>>>>>    performance, but I can't find a reference to it.
>>>>>
>>>>> I think the rationale is that ISR really needs to be PIO but everything
>>>>> else doesn't.  PIO is much faster on x86 because it doesn't require
>>>>> walking page tables or instruction emulation to handle the exit.
>>>>
>>>> Is this still a pressing issue?  With MSI-X enabled ISR isn't needed,
>>>> correct?  Which would imply that pretty much only old guests without
>>>> MSI-X support need this, and we don't need to worry that much when
>>>> designing something new ...
>>> 
>>> It wasn't that long ago that MSI-X wasn't supported..  I think we should
>>> continue to keep ISR as PIO as it is a fast path.
>>
>> No problem if we allow to have both legacy layout and new layout at the
>> same time.  Guests can continue to use ISR @ BAR0 in PIO space for
>> existing virtio devices, even in case they want use mmio for other
>> registers -> all fine.
>>
>> New virtio devices can support MSI-X from day one and decide to not
>> expose a legacy layout PIO bar.
>
> I think having BAR1 be an MMIO mirror of the registers + a BAR2 for
> virtio configuration space is probably not that bad of a solution.

Well, we also want to clean up the registers, so how about:

BAR0: legacy, as is.  If you access this, don't use the others.
BAR1: new format virtio-pci layout.  If you use this, don't use BAR0.
BAR2: virtio-cfg.  If you use this, don't use BAR0.
BAR3: ISR. If you use this, don't use BAR0.

I prefer the cases exclusive (ie. use one or the other) as a clear path
to remove the legacy layout; and leaving the ISR in BAR0 leaves us with
an ugly corner case in future (ISR is BAR0 + 19?  WTF?).

As to MMIO vs PIO, the BARs are self-describing, so we should explicitly
endorse that and leave it to the devices.

The detection is simple: if BAR1 has non-zero length, it's new-style,
otherwise legacy.

Thoughts?
Rusty.