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

[Anthony Liguori <aliguori@us.ibm.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> On Wed, Jun 05, 2013 at 03:42:57PM -0500, Anthony Liguori wrote:
>> "Michael S. Tsirkin" <mst@redhat.com> writes:
>> 
>> Can you explain?  I thought the whole trick with separating out the
>> virtqueue notification register was to regain the performance?
>
> Yes but this trick only works well with NPT (it's still a bit
> slower than PIO but not so drastically).
> Without NPT you still need a page walk so it will be slow.

Do you mean NPT/EPT?

If your concern is shadow paging, then I think you're concerned about
hardware that is so slow to start with that it's not worth considering.

>> >> It also maps to what regular hardware does.  I highly doubt that there
>> >> are any real PCI cards that made the shift from PCI to PCI-e without
>> >> bumping at least a revision ID.
>> >
>> > Only because the chance it's 100% compatible on the software level is 0.
>> > It always has some hardware specific quirks.
>> > No such excuse here.
>> >
>> >> It also means we don't need to play games about sometimes enabling IO
>> >> bars and sometimes not.
>> >
>> > This last paragraph is wrong, it ignores the issues 3) to 5) 
>> > I added above.
>> >
>> > If you do take them into account:
>> > 	- there are reasons to add MMIO BAR to PCI,
>> > 	  even without PCI express
>> 
>> So far, the only reason you've provided is "it doesn't work on some
>> architectures."  Which architectures?
>
> PowerPC wants this.

Existing PowerPC remaps PIO to MMAP so it works fine today.

Future platforms may not do this but future platforms can use a
different device.  They certainly won't be able to use the existing
drivers anyway.

Ben, am I wrong here?

>> > 	- we won't be able to drop IO BAR from virtio
>> 
>> An IO BAR is useless if it means we can't have more than 12 devices.
>
>
> It's not useless. A smart BIOS can enable devices one by one as
> it tries to boot from them.

A smart BIOS can also use MMIO to program virtio.

Regards,

Anthony Liguori