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

[Vikas Gupta]

Hi Alex,

On Mon, Nov 9, 2020 at 8:58 PM Alex Williamson
<alex.williamson@xxxxxxxxxx> wrote:
>
> On Mon, 9 Nov 2020 12:11:15 +0530
> Vikas Gupta <vikas.gupta@xxxxxxxxxxxx> wrote:
>
> > Hi Alex,
> >
> > On Fri, Nov 6, 2020 at 8:42 AM Alex Williamson
> > <alex.williamson@xxxxxxxxxx> wrote:
> > >
> > > On Fri, 6 Nov 2020 08:24:26 +0530
> > > Vikas Gupta <vikas.gupta@xxxxxxxxxxxx> wrote:
> > >
> > > > Hi Alex,
> > > >
> > > > On Thu, Nov 5, 2020 at 12:38 PM Alex Williamson
> > > > <alex.williamson@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Thu,  5 Nov 2020 11:32:55 +0530
> > > > > Vikas Gupta <vikas.gupta@xxxxxxxxxxxx> 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.
>
>
> It still seems questionable to me to use a device info capability to
> describe an interrupt index specific feature.  The scope seems wrong.
> Why does userspace need to know that this IRQ is MSI rather than
> indicating it's simply an edge triggered interrupt?  That can be done
> using only vfio_irq_info.flags.

Ok. In the next patch set I`ll remove the device flag (VFIO_DEVICE_FLAGS_MSI) as
vfio_irq_info.flags should have enough information for edge triggered interrupt.

>
>
> > > > 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.
>
> Indexes and sub-indexes are a grouping mechanism of vfio to describe
> related interrupts.  That terminology doesn't exist on PCI either, it's
> meant to be used generically.  It's left to the vfio bus driver how
> userspace associates a given index to a device feature.
>
> > 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?
>
>
> No, no-resize is an implementation detail, not an indication of the
> interrupt mechanism.  It's still not clear to me why it's important to
> expose to userspace that a given interrupt is MSI versus simply
> exposing it as an edge interrupt (ie. automasked = false).  If it is
> necessary, the most direct approach might be to expose a capability
> extension in the vfio_irq_info structure to describe it.  Even then
> though, I don't think simply exposing a index as MSI is very
> meaningful.  What is userspace intended to do differently based on this
> information?  Thanks,
The current patch set is not setting VFIO_IRQ_INFO_AUTOMASKED
(automasked=false) for MSIs so I believe this much is information
enough for user space to know that this is an edge triggered
interrupt.
 I agree that exposing an index as MSI is not meaningful as user space
has nothing special to do with this information.
>
> Alex
>

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