Re: [Qemu-devel] VFIO and scheduled SR-IOV cards

[Don Dutile]

On 06/03/2013 02:02 PM, Alex Williamson wrote:
> On Mon, 2013-06-03 at 18:33 +0200, Benoît Canet wrote:
> > Hello,
> >
> > I plan to write a PF driver for an SR-IOV card and make the VFs work with QEMU's
> > VFIO passthrough so I am asking the following design question before trying to
> > write and push code.
> >
> > After SR-IOV being enabled on this hardware only one VF function can be active
> > at a given time.
>
> Is this actually an SR-IOV device or are you trying to write a driver
> that emulates SR-IOV for a PF?
>
> > The PF host kernel driver is acting as a scheduler.
> > It switch every few milliseconds which VF is the current active function while
> > disabling the others VFs.
that's time-sharing of hw, which sw doesn't see ... so, ok.

> > One consequence of how the hardware works is that the MMR regions of the
> > switched off VFs must be unmapped and their io access should block until the VF
> > is switched on again.
This violates the spec., and does impact sw -- how can one assign such a VF to 
a guest
-- it does not work indep. of other VFs.

> MMR = Memory Mapped Register?
>
> This seems contradictory to the SR-IOV spec, which states:
>
>          Each VF contains a non-shared set of physical resources required
>          to deliver Function-specific
>          services, e.g., resources such as work queues, data buffers,
>          etc. These resources can be directly
>          accessed by an SI without requiring VI or SR-PCIM intervention.
>
> Furthermore, each VF should have a separate requester ID.  What's being
> suggested here seems like maybe that's not the case.  If true, it would
I didn't read it that way above.  I read it as the PCIe end is timeshared
btwn VFs (& PFs?). .... with some VFs disappearing (from a driver perspective)
as if the device was hot unplug w/o notification.  That will probably cause
read-timeouts & SME's, bringing down most enterprise-level systems.

> make iommu groups challenging.  Is there any VF save/restore around the
> scheduling?
>
> > Each IOMMU map/unmap should be done in less than 100ns.
>
> I think that may be a lot to ask if we need to unmap the regions in the
> guest and in the iommu.  If the "VFs" used different requester IDs,
> iommu unmapping whouldn't be necessary.  I experimented with switching
> between trapped (read/write) access to memory regions and mmap'd (direct
> mapping) for handling legacy interrupts.  There was a noticeable
> performance penalty switching per interrupt.
>
> > As the kernel iommu module is being called by the VFIO driver the PF driver
> > cannot interface with it.
> >
> > Currently the only interface of the VFIO code is for the userland QEMU process
> > and I fear that notifying QEMU that it should do the unmap/block would take more
> > than 100ns.
> >
> > Also blocking the IO access in QEMU under the BQL would freeze QEMU.
> >
> > Do you have and idea on how to write this required map and block/unmap feature ?
>
> It seems like there are several options, but I'm doubtful that any of
> them will meet 100ns.  If this is completely fake SR-IOV and there's not
> a different requester ID per VF, I'd start with seeing if you can even
> do the iommu_unmap/iommu_map of the MMIO BARs in under 100ns.  If that's
> close to your limit, then your only real option for QEMU is to freeze
> it, which still involves getting multiple (maybe many) vCPUs out of VM
> mode.  That's not free either.  If by some miracle you have time to
> spare, you could remap the regions to trapped mode and let the vCPUs run
> while vfio blocks on read/write.
>
> Maybe there's even a question whether mmap'd mode is worthwhile for this
> device.  Trapping every read/write is orders of magnitude slower, but
> allows you to handle the "wait for VF" on the kernel side.
>
> If you can provide more info on the device design/contraints, maybe we
> can come up with better options.  Thanks,
>
> Alex
>
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