Re: [PATCH for-9.2 v15 00/11] hw/pci: SR-IOV related fixes and improvements

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Sep 11, 2024 at 12:05:46PM +0900, Akihiko Odaki wrote:
> On 2024/09/11 0:27, Michael S. Tsirkin wrote:
> > On Tue, Sep 10, 2024 at 04:13:14PM +0200, Cédric Le Goater wrote:
> > > On 9/10/24 15:34, Michael S. Tsirkin wrote:
> > > > On Tue, Sep 10, 2024 at 03:21:54PM +0200, Cédric Le Goater wrote:
> > > > > On 9/10/24 11:33, Akihiko Odaki wrote:
> > > > > > On 2024/09/10 18:21, Michael S. Tsirkin wrote:
> > > > > > > On Fri, Aug 23, 2024 at 02:00:37PM +0900, Akihiko Odaki wrote:
> > > > > > > > Supersedes: <20240714-rombar-v2-0-af1504ef55de@daynix.com>
> > > > > > > > ("[PATCH v2 0/4] hw/pci: Convert rom_bar into OnOffAuto")
> > > > > > > > 
> > > > > > > > I submitted a RFC series[1] to add support for SR-IOV emulation to
> > > > > > > > virtio-net-pci. During the development of the series, I fixed some
> > > > > > > > trivial bugs and made improvements that I think are independently
> > > > > > > > useful. This series extracts those fixes and improvements from the RFC
> > > > > > > > series.
> > > > > > > > 
> > > > > > > > [1]: https://patchew.org/QEMU/20231210-sriov-v2-0-b959e8a6dfaf@daynix.com/
> > > > > > > > 
> > > > > > > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> > > > > > > 
> > > > > > > I don't think Cédric's issues have been addressed, am I wrong?
> > > > > > > Cédric, what is your take?
> > > > > > 
> > > > > > I put the URI to Cédric's report here:
> > > > > > https://lore.kernel.org/r/75cbc7d9-b48e-4235-85cf-49dacf3c7483@redhat.com
> > > > > > 
> > > > > > This issue was dealt with patch "s390x/pci: Check for multifunction after device realization". I found that s390x on QEMU does not support multifunction and SR-IOV devices accidentally circumvent this restriction, which means igb was never supposed to work with s390x. The patch prevents adding SR-IOV devices to s390x to ensure the restriction is properly enforced.
> > > > > 
> > > > > yes, indeed and it seems to fix :
> > > > > 
> > > > >     6069bcdeacee ("s390x/pci: Move some hotplug checks to the pre_plug handler")
> > > > > 
> > > > > I will update patch 4.
> > > > > 
> > > > > 
> > > > > Thanks,
> > > > > 
> > > > > C.
> > > > > 
> > > > > 
> > > > > That said, the igb device worked perfectly fine under the s390x machine.
> > > > 
> > > > And it works for you after this patchset, yes?
> > > 
> > > ah no, IGB is not an available device for the s390x machine anymore :
> > > 
> > >    qemu-system-s390x: -device igb,netdev=net1,mac=C0:FF:EE:00:00:13: multifunction not supported in s390
> > 
> > 
> > So patch 4 didn't relly help.
> > 
> > 
> > > This is what commit 57da367b9ec4 ("s390x/pci: forbid multifunction
> > > pci device") initially required (and later broken by 6069bcdeacee).
> > > So I guess we are fine with the expected behavior.
> > > 
> > > Thanks,
> > > 
> > > C.
> > 
> > Better to fix than to guess if there are users, I think.
> 
> Yes, but it will require some knowledge of s390x, which I cannot provide.
> 
> Commit 57da367b9ec4 ("s390x/pci: forbid multifunction pci device") says
> having a multifunction device will make the guest spin forever. That is not
> what Cédric observed with igb so it may no longer be relevant, but I cannot
> be sure that the problem is resolved without understanding how multifunction
> devices are supposed to work with s390x.
> 
> Ideally someone with s390x expertise should check relevant hardware
> documentation and confirm QEMU properly implements mutlifunction devices.

The fact is, QEMU already does what most users want from it. So the
first rule whenever adding a new feature is not breaking old
functionality.  I know, it's annoying, as some of it is held together by
duct tape.  We have a friendly community so if you ask nicely, you
usually can get help. You'd probably have to be a bit more specific
with your questions.

> Let's keep the restriction until then.
> 
> Regards,
> Akihiko Odaki


Not introducing regressions is a hard rule, sorry.

-- 
MST