RE: [PATCH v2 2/4] vfio/pci: enable vector on dynamic MSI-X allocation

[Liu, Jing2]

Hi Alex,

> On Sat, 9/23/2023 4:55 AM, Alex Williamson <alex.williamson@redhat.com> wrote:
> On Mon, 18 Sep 2023 05:45:05 -0400
> Jing Liu <jing2.liu@intel.com> wrote:
> 
> > The vector_use callback is used to enable vector that is unmasked in
> > guest. The kernel used to only support static MSI-X allocation. When
> > allocating a new interrupt using "static MSI-X allocation" kernels,
> > QEMU first disables all previously allocated vectors and then
> > re-allocates all including the new one. The nr_vectors of
> > VFIOPCIDevice indicates that all vectors from 0 to nr_vectors are
> > allocated (and may be enabled), which is used to to loop all the
> > possibly used vectors
>                              ^^ ^^
> 
> s/to to/to/

Will change.
> 
> > When, e.g., disabling MSI-X interrupts.
> >
> > Extend the vector_use function to support dynamic MSI-X allocation
> > when host supports the capability. QEMU therefore can individually
> > allocate and enable a new interrupt without affecting others or
> > causing interrupts lost during runtime.
> >
> > Utilize nr_vectors to calculate the upper bound of enabled vectors in
> > dynamic MSI-X allocation mode since looping all msix_entries_nr is not
> > efficient and unnecessary.
> >
> > Signed-off-by: Jing Liu <jing2.liu@intel.com>
> > Tested-by: Reinette Chatre <reinette.chatre@intel.com>
> > ---
> > Changes since v1:
> > - Revise Qemu to QEMU.
> >
> > Changes since RFC v1:
> > - Test vdev->msix->noresize to identify the allocation mode. (Alex)
> > - Move defer_kvm_irq_routing test out and update nr_vectors in a
> >   common place before vfio_enable_vectors(). (Alex)
> > - Revise the comments. (Alex)
> > ---
> >  hw/vfio/pci.c | 44 +++++++++++++++++++++++++++-----------------
> >  1 file changed, 27 insertions(+), 17 deletions(-)
> >
> > diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c index
> > 60654ca28ab8..84987e46fd7a 100644
> > --- a/hw/vfio/pci.c
> > +++ b/hw/vfio/pci.c
> > @@ -470,6 +470,7 @@ static int vfio_msix_vector_do_use(PCIDevice *pdev,
> unsigned int nr,
> >      VFIOPCIDevice *vdev = VFIO_PCI(pdev);
> >      VFIOMSIVector *vector;
> >      int ret;
> > +    int old_nr_vecs = vdev->nr_vectors;
> 
> Minor suggestion, it reads slightly better below if this were something
> like:
> 
>     bool resizing = !!(vdev->nr_vectors < nr + 1);
> 
> Then use the bool in place of the nr+1 tests below.  Thanks,
> 
Got it. This change makes it nice to read. Thanks for the advice. Will send v3 
later. 

Thanks,
Jing

> Alex
> 
> >
> >      trace_vfio_msix_vector_do_use(vdev->vbasedev.name, nr);
> >
> > @@ -512,33 +513,42 @@ static int vfio_msix_vector_do_use(PCIDevice
> *pdev, unsigned int nr,
> >      }
> >
> >      /*
> > -     * We don't want to have the host allocate all possible MSI vectors
> > -     * for a device if they're not in use, so we shutdown and incrementally
> > -     * increase them as needed.
> > +     * When dynamic allocation is not supported, we don't want to have the
> > +     * host allocate all possible MSI vectors for a device if they're not
> > +     * in use, so we shutdown and incrementally increase them as needed.
> > +     * nr_vectors represents the total number of vectors allocated.
> > +     *
> > +     * When dynamic allocation is supported, let the host only allocate
> > +     * and enable a vector when it is in use in guest. nr_vectors 
> > represents
> > +     * the upper bound of vectors being enabled (but not all of the ranges
> > +     * is allocated or enabled).
> >       */
> >      if (vdev->nr_vectors < nr + 1) {
> >          vdev->nr_vectors = nr + 1;
> > -        if (!vdev->defer_kvm_irq_routing) {
> > +    }
> > +
> > +    if (!vdev->defer_kvm_irq_routing) {
> > +        if (vdev->msix->noresize && (old_nr_vecs < nr + 1)) {
> >              vfio_disable_irqindex(&vdev->vbasedev, 
> > VFIO_PCI_MSIX_IRQ_INDEX);
> >              ret = vfio_enable_vectors(vdev, true);
> >              if (ret) {
> >                  error_report("vfio: failed to enable vectors, %d", ret);
> >              }
> > -        }
> > -    } else {
> > -        Error *err = NULL;
> > -        int32_t fd;
> > -
> > -        if (vector->virq >= 0) {
> > -            fd = event_notifier_get_fd(&vector->kvm_interrupt);
> >          } else {
> > -            fd = event_notifier_get_fd(&vector->interrupt);
> > -        }
> > +            Error *err = NULL;
> > +            int32_t fd;
> >
> > -        if (vfio_set_irq_signaling(&vdev->vbasedev,
> > -                                     VFIO_PCI_MSIX_IRQ_INDEX, nr,
> > -                                     VFIO_IRQ_SET_ACTION_TRIGGER, fd, 
> > &err)) {
> > -            error_reportf_err(err, VFIO_MSG_PREFIX, vdev->vbasedev.name);
> > +            if (vector->virq >= 0) {
> > +                fd = event_notifier_get_fd(&vector->kvm_interrupt);
> > +            } else {
> > +                fd = event_notifier_get_fd(&vector->interrupt);
> > +            }
> > +
> > +            if (vfio_set_irq_signaling(&vdev->vbasedev,
> > +                                       VFIO_PCI_MSIX_IRQ_INDEX, nr,
> > +                                       VFIO_IRQ_SET_ACTION_TRIGGER, fd, 
> > &err)) {
> > +                error_reportf_err(err, VFIO_MSG_PREFIX, 
> > vdev->vbasedev.name);
> > +            }
> >          }
> >      }
> >