Re: [Qemu-devel] [PATCH v2] pci: relax pci_msi_get_message()

[Peter Xu]

On Fri, Nov 25, 2016 at 06:11:01AM +0200, Michael S. Tsirkin wrote:
> On Tue, Nov 22, 2016 at 04:08:50PM +0800, Peter Xu wrote:
> > We are very strict in the past getting MSIs from commit
> > d1f6af6a1 ("kvm-irqchip: simplify kvm_irqchip_add_msi_route"), assuming
> > that MSI should be configured before hand when fetching. When we have
> > unrecognized configurations, we panic the system. However looks like
> > this is too strict to be working on some platform, and issues occured.
> > Firstly it's found on a ppc case and fixed by David in:
> > 
> >   6d17a01 vfio/pci: Fix regression in MSI routing configuration
> > 
> > However we encountered another case now with windows virtio driver and
> > reported (and possibly more):
> > 
> >   http://bugs.debian.org/844361
> > 
> > To make every driver/hardware happy, let's loosen the rule and go back
> > to the original behavior - instead of panic the system, when we try to
> > fetch MSI without configured MSI/MSI-X system, we just provide an empty
> > message to make drivers happy.
> > 
> > Reported-by: Maciej Kotliński <address@hidden>
> > Signed-off-by: Peter Xu <address@hidden>
> > ---
> >  hw/pci/pci.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> > index 24fae16..0cd2bb0 100644
> > --- a/hw/pci/pci.c
> > +++ b/hw/pci/pci.c
> > @@ -2606,9 +2606,11 @@ MSIMessage pci_get_msi_message(PCIDevice *dev, int 
> > vector)
> >      } else if (msi_enabled(dev)) {
> >          msg = msi_get_message(dev, vector);
> >      } else {
> > -        /* Should never happen */
> > -        error_report("%s: unknown interrupt type", __func__);
> > -        abort();
> > +        /*
> > +         * Device is not configured with MSI/MSI-X yet, let's provide
> > +         * an empty message to make all device drivers happy.
> > +         */
> > +        memset(&msg, 0, sizeof(msg));
> >      }
> >      return msg;
> >  }
> 
> Looks like a hack to me. Even if callers happen to treat
> 0 message as a nop, there's no guarantee that's true for
> all platforms. Pls change pci_get_msi_message to
> return an error code, and fix callers to check that.

Actually I'd say I still cannot understand why some guests will
encounter this issue - the driver just tries to fetch and enable
MSI/MSI-X messages without fully activate MSI/MSI-X in general (that's
why msi_enabled() and msix_enabled() both returned false).

But indeed some drivers won't work properly after commit d1f6af6a.
David fixed some case for ppc and vfio (6d17a01), but problem still
exist, like Debian's report (http://bugs.debian.org/844361).

To avoid going into every details of guest drivers, I was thinking
maybe we can just "recover" the behavior to the past: old QEMU (before
d1f6af6a) allows to fetch an meaningless message (e.g., when
msi_get_message() is called, it's possible that MSI is still not
setup, but IMHO that message is meaningless). So here we emulate that
case, and return a meaningless message. From the Debian bug report, we
can see that this does solve the issue (yeah I know this is not even
an excuse to agree on this patch, but again I just think this is
currently the best way to solve the problem...).

If we change this into return error, then kvm_irqchip_add_msi_route()
might fail, and that's a different behavior again, I don't know
whether it'll bring more troubles (only if I can test every guest
driver with that code, but that's merely impossible). So IMHO the best
way to solve the problem is use the current patch and try to keep the
old behavior.

I would really appreciate if you (or anyone) has better suggestion.

Thanks,

-- peterx