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 07:59:33AM -0500, Anthony Liguori wrote:
>> "Michael S. Tsirkin" <mst@redhat.com> writes:
>> 
>> > On Tue, Jun 04, 2013 at 03:01:50PM +0930, Rusty Russell wrote:
>> > You mean make BAR0 an MMIO BAR?
>> > Yes, it would break current windows guests.
>> > Further, as long as we use same address to notify all queues,
>> > we would also need to decode the instruction on x86 and that's
>> > measureably slower than PIO.
>> > We could go back to discussing hypercall use for notifications,
>> > but that has its own set of issues...
>> 
>> So... does "violating the PCI-e" spec really matter?  Is it preventing
>> any guest from working properly?
>
> Yes, absolutely, this wording in spec is not there without reason.
>
> Existing guests allocate io space for PCI express ports in
> multiples on 4K.
>
> Since each express device is behind such a port, this means
> at most 15 such devices can use IO ports in a system.
>
> That's why to make a pci express virtio device,
> we must allow MMIO and/or some other communication
> mechanism as the spec requires.

This is precisely why this is an ABI breaker.

If you disable IO bars in the BIOS, than the interface that the OS sees
will *not have an IO bar*.

This *breaks existing guests*.

Any time the programming interfaces changes on a PCI device, the
revision ID and/or device ID must change.  The spec is very clear about
this.

We cannot disable the IO BAR without changing revision ID/device ID.

> That's on x86.
>
> Besides x86, there are achitectures where IO is unavailable or very slow.
>
>> I don't think we should rush an ABI breakage if the only benefit is
>> claiming spec compliance.
>> 
>> Regards,
>> 
>> Anthony Liguori
>
> Why do you bring this up? No one advocates any ABI breakage,
> I only suggest extensions.

It's an ABI breakage.  You're claiming that the guests you tested
handle the breakage reasonably but it is unquestionably an ABI breakage.

Regards,

Anthony Liguori

>
>
>> >
>> > -- 
>> > MST
>> > --
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