Re: [PATCH v10 13/14] vfio-user: handle device interrupts

[Stefan Hajnoczi]

On Tue, 31 May 2022 at 21:11, Alex Williamson
<alex.williamson@redhat.com> wrote:
>
> On Tue, 31 May 2022 15:01:57 +0000
> Jag Raman <jag.raman@oracle.com> wrote:
>
> > > On May 25, 2022, at 10:53 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > >
> > > On Tue, May 24, 2022 at 11:30:32AM -0400, Jagannathan Raman wrote:
> > >> Forward remote device's interrupts to the guest
> > >>
> > >> Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
> > >> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> > >> Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
> > >> ---
> > >> include/hw/pci/pci.h              |  13 ++++
> > >> include/hw/remote/vfio-user-obj.h |   6 ++
> > >> hw/pci/msi.c                      |  16 ++--
> > >> hw/pci/msix.c                     |  10 ++-
> > >> hw/pci/pci.c                      |  13 ++++
> > >> hw/remote/machine.c               |  14 +++-
> > >> hw/remote/vfio-user-obj.c         | 123 ++++++++++++++++++++++++++++++
> > >> stubs/vfio-user-obj.c             |   6 ++
> > >> MAINTAINERS                       |   1 +
> > >> hw/remote/trace-events            |   1 +
> > >> stubs/meson.build                 |   1 +
> > >> 11 files changed, 193 insertions(+), 11 deletions(-)
> > >> create mode 100644 include/hw/remote/vfio-user-obj.h
> > >> create mode 100644 stubs/vfio-user-obj.c
> > >
> > > It would be great if Michael Tsirkin and Alex Williamson would review
> > > this.
> >
> > Hi Michael and Alex,
> >
> > Do you have any thoughts on this patch?
>
> Ultimately this is just how to insert callbacks to replace the default
> MSI/X triggers so you can send a vector# over the wire for a remote
> machine, right?  I'll let the code owners, Michael and Marcel, comment
> if they have grand vision how to architect this differently.  Thanks,

An earlier version of the patch intercepted MSI-X at the msix_notify()
level, replacing the entire function. This patch replaces
msix_get_message() and msi_send_message(), leaving the masking logic
in place.

I haven't seen the latest vfio-user client implementation for QEMU,
but if the idea is to allow the guest to directly control the
vfio-user device's MSI-X table's mask bits, then I think this is a
different design from VFIO kernel where masking is emulated by QEMU
and not passed through to the PCI device.

It's been a while since I looked at how this works in QEMU's hw/vfio/
code, so I may not be explaining it correctly, but I think there is a
design difference here between VFIO kernel and vfio-user that's worth
evaluating.

Stefan