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

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Date:   Tue, 10 Nov 2020 16:31:23 +0530
From:   Vikas Gupta <vikas.gupta@...adcom.com>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     Alex Williamson <alex.williamson@...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 Eric,

On Mon, Nov 9, 2020 at 8:35 PM Auger Eric <eric.auger@...hat.com> wrote:
>
> Hi Vikas,
>
> On 11/6/20 3:54 AM, Vikas Gupta 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.
> > In the patch set there is no difference between MSI and normal
> > interrupt for VFIO_DEVICE_GET_IRQ_INFO.
> in vfio_platform_irq_init() we first iterate on normal interrupts using
> get_irq(). Can't we add an MSI index at the end of this list with
> vdev->irqs[i].count > 1 and set vdev->num_irqs accordingly?
Yes, I think MSI can be added to the end of list with setting
vdev->irqs[i].count > 1.
I`ll consider changing in the next patch set.
Thanks,
Vikas
>
> Thanks
>
> Eric
> > 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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