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

[Vikas Gupta <vikas.gupta@broadcom.com>]

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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.
In the patch set there is no difference between MSI and normal
interrupt for VFIO_DEVICE_GET_IRQ_INFO.
The patch set adds MSI(s), say as an extension, to the normal
interrupts and handled accordingly. Do you see this is a violation? If
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.

Thanks,
Vikas

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