Re: [RFC] /dev/ioasid uAPI proposal

[Jean-Philippe Brucker]

On Tue, Jun 08, 2021 at 04:31:50PM +1000, David Gibson wrote:
> For the qemu case, I would imagine a two stage fallback:
> 
>     1) Ask for the exact IOMMU capabilities (including pagetable
>        format) that the vIOMMU has.  If the host can supply, you're
>        good
> 
>     2) If not, ask for a kernel managed IOAS.  Verify that it can map
>        all the IOVA ranges the guest vIOMMU needs, and has an equal or
>        smaller pagesize than the guest vIOMMU presents.  If so,
>        software emulate the vIOMMU by shadowing guest io pagetable
>        updates into the kernel managed IOAS.
> 
>     3) You're out of luck, don't start.
>     
> For both (1) and (2) I'd expect it to be asking this question *after*
> saying what devices are attached to the IOAS, based on the virtual
> hardware configuration.  That doesn't cover hotplug, of course, for
> that you have to just fail the hotplug if the new device isn't
> supportable with the IOAS you already have.

Yes. So there is a point in time when the IOAS is frozen, and cannot take
in new incompatible devices. I think that can support the usage I had in
mind. If the VMM (non-QEMU, let's say) wanted to create one IOASID FD per
feature set it could bind the first device, freeze the features, then bind
the second device. If the second bind fails it creates a new FD, allowing
to fall back to (2) for the second device while keeping (1) for the first
device. A paravirtual IOMMU like virtio-iommu could easily support this as
it describes pIOMMU properties for each device to the guest. An emulated
vIOMMU could also support some hybrid cases as you describe below.

> One can imagine optimizations where for certain intermediate cases you
> could do a lighter SW emu if the host supports a model that's close to
> the vIOMMU one, and you're able to trap and emulate the differences.
> In practice I doubt anyone's going to have time to look for such cases
> and implement the logic for it.
> 
> > For example depending whether the hardware IOMMU is SMMUv2 or SMMUv3, that
> > completely changes the capabilities offered to the guest (some v2
> > implementations support nesting page tables, but never PASID nor PRI
> > unlike v3.) The same vIOMMU could support either, presenting different
> > capabilities to the guest, even multiple page table formats if we wanted
> > to be exhaustive (SMMUv2 supports the older 32-bit descriptor), but it
> > needs to know early on what the hardware is precisely. Then some new page
> > table format shows up and, although the vIOMMU can support that in
> > addition to older ones, QEMU will have to pick a single one, that it
> > assumes the guest knows how to drive?
> > 
> > I think once it binds a device to an IOASID fd, QEMU will want to probe
> > what hardware features are available before going further with the vIOMMU
> > setup (is there PASID, PRI, which page table formats are supported,
> > address size, page granule, etc). Obtaining precise information about the
> > hardware would be less awkward than trying different configurations until
> > one succeeds. Binding an additional device would then fail if its pIOMMU
> > doesn't support exactly the features supported for the first device,
> > because we don't know which ones the guest will choose. QEMU will have to
> > open a new IOASID fd for that device.
> 
> No, this fundamentally misunderstands the qemu model.  The user
> *chooses* the guest visible platform, and qemu supplies it or fails.
> There is no negotiation with the guest, because this makes managing
> migration impossibly difficult.

I'd like to understand better where the difficulty lies, with migration.
Is the problem, once we have a guest running on physical machine A, to
make sure that physical machine B supports the same IOMMU properties
before migrating the VM over to B?  Why can't QEMU (instead of the user)
select a feature set on machine A, then when time comes to migrate, query
all information from the host kernel on machine B and check that it
matches what was picked for machine A?  Or is it only trying to
accommodate different sets of features between A and B, that would be too
difficult?

Thanks,
Jean

> 
> -cpu host is an exception, which is used because it is so useful, but
> it's kind of a pain on the qemu side.  Virt management systems like
> oVirt/RHV almost universally *do not use* -cpu host, precisely because
> it cannot support predictable migration.
> 
> -- 
> David Gibson			| I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
> 				| _way_ _around_!
> http://www.ozlabs.org/~dgibson