RE: [PATCH v2 10/10] vfio/pci: Add VFIO_DEVICE_GET_PCI_HOT_RESET_GROUP_INFO

Liu, Yi L
2023-03-28T15:02:02Z

> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Tuesday, March 28, 2023 10:46 PM
>
> On Tue, 28 Mar 2023 14:38:12 +0000
> "Liu, Yi L" <yi.l.liu@intel.com> wrote:
>
> > > From: Alex Williamson <alex.williamson@redhat.com>
> > > Sent: Tuesday, March 28, 2023 10:26 PM
> > >
> > > On Tue, 28 Mar 2023 06:19:06 +0000
> > > "Tian, Kevin" <kevin.tian@intel.com> wrote:
> > >
> > > > > From: Liu, Yi L <yi.l.liu@intel.com>
> > > > > Sent: Tuesday, March 28, 2023 11:32 AM
> > > > >
> > > > > > From: Alex Williamson <alex.williamson@redhat.com>
> > > > > > Sent: Tuesday, March 28, 2023 3:26 AM
> > > > > >
> > > > > > Additionally, VFIO_DEVICE_GET_PCI_HOT_RESET_INFO has a flags
> arg
> > > that
> > > > > > isn't used, why do we need a new ioctl vs defining
> > > > > > VFIO_PCI_HOT_RESET_FLAG_IOMMUFD_DEV_ID.
> > > > >
> > > > > Sure. I can follow this suggestion. BTW. I have a doubt here. This
> new
> > > flag
> > > > > is set by user. What if in the future kernel has new extensions and
> needs
> > > > > to report something new to the user and add new flags to tell user?
> Such
> > > > > flag is set by kernel. Then the flags field may have two kinds of flags
> > > (some
> > > > > set by user while some set by kernel). Will it mess up the flags space?
> > > > >
> > > >
> > > > flags in a GET_INFO ioctl is for output.
> > > >
> > > > if user needs to use flags as input to select different type of info then it
> > > should
> > > > be split into multiple GET_INFO cmds.
> > >
> > > I don't know that that's actually a rule, however we don't currently
> > > test flags is zero for input, so in this case I think we are stuck with
> > > it only being for output.
> > >
> > > Alternatively, should VFIO_DEVICE_GET_PCI_HOT_RESET_INFO
> > > automatically
> > > return the dev_id variant of the output and set a flag to indicate this
> > > is the case when called on a device fd opened as a cdev? Thanks,
> >
> > Personally I prefer that user asks for dev_id info explicitly. The major
> reason
> > that we return dev_id is that the group/bdf info is not enough for the
> device
> > fd passing case. But if qemu opens device by itself, the group/bdf info is
> still
> > enough. So a device opened as a cdev doesn't mean it should return
> dev_id,
> > it depends on if user has the bdf knowledge.
>
> But if QEMU opens the cdev, vs getting it from the group, does it make
> any sense to return a set of group-ids + bdf in the host-reset info?
> I'm inclined to think the answer is no.
>
> Per my previous suggestion, I think we should always return the bdf. We
> can't know if the user is accessing through an fd they opened
> themselves or were passed,

Oh, yes. I'm convinced by this reason since only cdev mode supports device fd
passing. So I'll reuse the existing _INFO and let kernel set a flag to mark the returned
info is dev_id+bdf.

A check. If the device that the _INFIO is invoked is opened via cdev, but there
are devices in the dev_set that are got via VFIO_GROUP_GET_DEVICE_FD, should
I fail it or allow it?

> but it allows an actually useful debugging
> report if userspace can know "I can't perform a hot reset of this
> device because my iommufd context doesn't know about device <bdf>", vs
> an opaque -EPERM. Therefore I think we're only discussing the data
> conveyed in the u32, a group-id or dev_id. Thanks,

Sure.

Regards,
Yi Liu