Re: [PATCH] qrap: Pass PCI device numbers to qemu in base 10, not in base 16

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 24 Feb 2023 11:05:00 -0800
Andrea Bolognani <abologna@redhat.com> wrote:

> On Fri, Feb 24, 2023 at 08:14:16AM +0100, Stefano Brivio wrote:
> > On Thu, 23 Feb 2023 06:06:17 -0800 Andrea Bolognani <abologna@redhat.com> wrote:  
> > > On Thu, Feb 23, 2023 at 09:27:14AM +1100, David Gibson wrote:  
> > > > On Wed, Feb 22, 2023 at 02:40:32AM -0800, Andrea Bolognani wrote:  
> > > > > I don't think this is going to work.
> > > > >
> > > > > The problem is that, while PCI buses are indeed named with increasing
> > > > > numbers in integer format (pci.9, pci.10 and so on), PCI slots are
> > > > > addressed using hexadecimal format (0x9, 0xa and so on). libvirt uses
> > > > > this naming convention because it matches QEMU's.  
> > > >
> > > > Actually, I think we're ok.  PCI slots are addressed in hex by
> > > > convention, but AFAICT if you *just* give a slot number, it will
> > > > accept either decimal or hex (so addr=10 and addr=0xa are equivalent).
> > > > That's *not* true if you use SS.F format to include the function
> > > > number - then it expects hex only.  But we're not doing that, so so
> > > > always using decimal should be ok here.
> > > >
> > > > Source: set_pci_devfn() in the qemu source
> > > >
> > > > Obviously that's a pretty fragile hack, but that's 'qrap' for you.  
> > >
> > > Yeah, even if that happens to work I'd rather not rely on it,
> > > especially since a proper solution doesn't look like it would be a
> > > lot of additional effort.
> > >
> > > I've managed to reproduce the original issue in the context of
> > > KubeVirt. I'll hopefully have a patch ready soon.  
> >
> > Andrea, allow me to do this: I would push this patch meanwhile, along
> > with the changes for the DNS issue you reported, because that one might
> > impact many users, and I think it makes sense to have a fix out soon.
> >
> > I start thinking it's also part of the issue Paul reported for Podman
> > with pasta here:
> >   https://github.com/containers/podman/issues/17074
> >
> > This patch itself can't hurt, and it changes exactly two letters.  
> 
> I strongly disagree with this assessment. This patch merely trades
> one set of issues for another one.
> 
> In particular, for pc machine types we'd end up producing
> bus=pci.0,addr=0x10 for slot 10 instead of bus=pci.0,addr=0xa,
> because the addr=0x part is baked into the template. So the QEMU
> logic David mentioned above wouldn't kick in at all.
> 
> More importantly, for q35 machines we'd start producing decimal bus
> numbers while still parsing the ones present in the original command
> line as hexadecimal, so things would stop lining up as soon as enough
> devices are present, meaning that the issue reported by Alona would
> still exist.

Oh, okay, sorry, I thought you and David agreed that it actually
happens to work. But anyway, nice that it doesn't matter now. :)

> > As soon as you have something less qrappy we'll go with that (you don't
> > even need to rebase, I'll revert this one on the tree first).  
> 
> Patches fixing this issue, as well as a few additional ones, are now
> on the list. I'll follow up on that thread with some considerations
> related to testing the changes.

Thanks a lot!

I didn't push out commits or a release today because I hit a false
positive with cppcheck 2.10 (cppcheck bisected but still trying to
grasp the issue), so that will all be for the next week I guess.

-- 
Stefano