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

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Message-ID: <CAHLZf_uan_nbewxRgTtbkAAk1rGAggq_3Z4EgtRqsPryt58eOw@mail.gmail.com>
Date:   Mon, 9 Nov 2020 12:11:15 +0530
From:   Vikas Gupta <vikas.gupta@...adcom.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Auger Eric <eric.auger@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Vikram Prakash <vikram.prakash@...adcom.com>
Subject: Re: [RFC, v0 1/3] vfio/platform: add support for msi

Hi Alex,

On Fri, Nov 6, 2020 at 8:42 AM Alex Williamson
<alex.williamson@...hat.com> wrote:
>
> On Fri, 6 Nov 2020 08:24:26 +0530
> Vikas Gupta <vikas.gupta@...adcom.com> wrote:
>
> > Hi Alex,
> >
> > On Thu, Nov 5, 2020 at 12:38 PM Alex Williamson
> > <alex.williamson@...hat.com> wrote:
> > >
> > > On Thu,  5 Nov 2020 11:32:55 +0530
> > > Vikas Gupta <vikas.gupta@...adcom.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?

No, it's not indicating that all are MSI.
The rationale behind adding the flag to tell user-space that platform
device supports MSI as well. As you mentioned recently added
capabilities can help on this, I`ll go through that.

>
> > 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?

With this patch set, I think this is missing and user space cannot
know that particular index is MSI interrupt.
For platform devices there is no such mechanism, like index and
sub-indexes to differentiate between legacy, MSI or MSIX as it’s there
in PCI.
I believe for a particular IRQ index if the flag
VFIO_IRQ_INFO_NORESIZE is used then user space can know which IRQ
index has MSI(s). Does it make sense?
Suggestions on this would be helpful.

Thanks,
Vikas
>
> > 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
>

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