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

[Gerd Hoffmann <kraxel@redhat.com>]

  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.

cheers,
  Gerd