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Date:   Thu, 5 Nov 2020 20:12:08 -0700
From:   Alex Williamson <alex.williamson@redhat.com>
To:     Vikas Gupta <vikas.gupta@broadcom.com>
Cc:     Auger Eric <eric.auger@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Vikram Prakash <vikram.prakash@broadcom.com>
Subject: Re: [RFC, v0 1/3] vfio/platform: add support for msi
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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