Re: [RFC, v0 1/3] vfio/platform: add support for msi

Alex Williamson
2020-11-06T03:12:21Z

On Fri, 6 Nov 2020 08:24:26 +0530
Vikas Gupta <vikas.gupta@broadcom.com> wrote:

> Hi Alex,
>
> On Thu, Nov 5, 2020 at 12:38 PM Alex Williamson
> <alex.williamson@redhat.com> wrote:
> >
> > On Thu, 5 Nov 2020 11:32:55 +0530
> > Vikas Gupta <vikas.gupta@broadcom.com> wrote:
> >
> > > diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> > > index 2f313a238a8f..aab051e8338d 100644
> > > --- a/include/uapi/linux/vfio.h
> > > +++ b/include/uapi/linux/vfio.h
> > > @@ -203,6 +203,7 @@ struct vfio_device_info {
> > > #define VFIO_DEVICE_FLAGS_AP (1 << 5) /* vfio-ap device */
> > > #define VFIO_DEVICE_FLAGS_FSL_MC (1 << 6) /* vfio-fsl-mc device */
> > > #define VFIO_DEVICE_FLAGS_CAPS (1 << 7) /* Info supports caps */
> > > +#define VFIO_DEVICE_FLAGS_MSI (1 << 8) /* Device supports msi */
> > > __u32 num_regions; /* Max region index + 1 */
> > > __u32 num_irqs; /* Max IRQ index + 1 */
> > > __u32 cap_offset; /* Offset within info struct of first cap */
> >
> > This doesn't make any sense to me, MSIs are just edge triggered
> > interrupts to userspace, so why isn't this fully described via
> > VFIO_DEVICE_GET_IRQ_INFO? If we do need something new to describe it,
> > this seems incomplete, which indexes are MSI (IRQ_INFO can describe
> > that)? We also already support MSI with vfio-pci, so a global flag for
> > the device advertising this still seems wrong. Thanks,
> >
> > Alex
> >
> Since VFIO platform uses indexes for IRQ numbers so I think MSI(s)
> cannot be described using indexes.

That would be news for vfio-pci which has been describing MSIs with
sub-indexes within indexes since vfio started.

> In the patch set there is no difference between MSI and normal
> interrupt for VFIO_DEVICE_GET_IRQ_INFO.

Then what exactly is a global device flag indicating? Does it indicate
all IRQs are MSI?

> The patch set adds MSI(s), say as an extension, to the normal
> interrupts and handled accordingly.

So we have both "normal" IRQs and MSIs? How does the user know which
indexes are which?

> Do you see this is a violation? If

Seems pretty unclear and dubious use of a global device flag.

> yes, then we`ll think of other possible ways to support MSI for the
> platform devices.
> Macro VFIO_DEVICE_FLAGS_MSI can be changed to any other name if it
> collides with an already supported vfio-pci or if not necessary, we
> can remove this flag.

If nothing else you're using a global flag to describe a platform
device specific augmentation. We've recently added capabilities on the
device info return that would be more appropriate for this, but
fundamentally I don't understand why the irq info isn't sufficient.
Thanks,

Alex