Re: [Qemu-ppc] [RFC PATCH qemu v3 4/4] vfio: spapr: Add SPAPR IOMMU v2 support (DMA memory preregistering)

[Alex Williamson]

On Fri, 2015-07-17 at 15:20 +1000, David Gibson wrote:
> On Thu, Jul 16, 2015 at 08:44:59AM -0600, Alex Williamson wrote:
> > On Thu, 2015-07-16 at 15:11 +1000, David Gibson wrote:
> > > On Tue, Jul 14, 2015 at 10:21:54PM +1000, Alexey Kardashevskiy wrote:
> > > > This makes use of the new "memory registering" feature. The idea is
> > > > to provide the userspace ability to notify the host kernel about pages
> > > > which are going to be used for DMA. Having this information, the host
> > > > kernel can pin them all once per user process, do locked pages
> > > > accounting (once) and not spent time on doing that in real time with
> > > > possible failures which cannot be handled nicely in some cases.
> > > > 
> > > > This adds a guest RAM memory listener which notifies a VFIO container
> > > > about memory which needs to be pinned/unpinned. VFIO MMIO regions
> > > > (i.e. "skip dump" regions) are skipped.
> > > > 
> > > > The feature is only enabled for SPAPR IOMMU v2. The host kernel changes
> > > > are required. Since v2 does not need/support VFIO_IOMMU_ENABLE, this 
> > > > does
> > > > not call it when v2 is detected and enabled.
> > > > 
> > > > This does not change the guest visible interface.
> > > > 
> > > > Signed-off-by: Alexey Kardashevskiy <address@hidden>
> > > 
> > > I've looked at this in more depth now, and attempting to unify the
> > > pre-reg and mapping listeners like this can't work - they need to be
> > > listening on different address spaces:  mapping actions need to be
> > > listening on the PCI address space, whereas the pre-reg needs to be
> > > listening on address_space_memory.  For x86 - for now - those end up
> > > being the same thing, but on Power they're not.
> > > 
> > > We do need to be clear about what differences are due to the presence
> > > of a guest IOMMU versus which are due to arch or underlying IOMMU
> > > type.  For now Power has a guest IOMMU and x86 doesn't, but that could
> > > well change in future: we could well implement the guest side IOMMU
> > > for x86 in future (or x86 could invent a paravirt IOMMU interface).
> > > On the other side, BenH's experimental powernv machine type could
> > > introduce Power machines without a guest side IOMMU (or at least an
> > > optional guest side IOMMU).
> > > 
> > > The quick and dirty approach here is:
> > >    1. Leave the main listener as is
> > >    2. Add a new pre-reg notifier to the spapr iommu specific code,
> > >       which listens on address_space_memory, *not* the PCI space
> > 
> > It is dirty and that's exactly what I've been advising Alexey against
> > because we have entirely too much dirty spapr specific code that doesn't
> > need to be spapr specific.  I don't see why separate address space
> > matters, that's done at the point of registering the listener and so far
> > doesn't play much a role in the actual listener behavior, just which
> > regions it sees.
> 
> Well, there's two parts to this - the different address spaces means
> they need to be different listener instances.  They also need
> different callbacks - or at least parameterized callback behaviour
> because they do different things (one maps, the other preregs).

Yep, and Alexey's doing that with a switch statement, which seems like
it's helping to share setup w/o being too ugly.  Whether it's a "map" or
a "pre-reg", we don't really care, it's just an ioctl specific to the
vfio-iommu type with some setup.  Much of that setup is not specific to
the vfio-iommu type and can be shared between guest-iommu type,
transparent or visible.  That's the part were I do not want spapr
specific setup hiding what should be generic code for anybody that has a
guest-visible iommu versus guest-transparent iommu.

> So I really don't see any sense in which these can be accomplished by
> the same listener.  *Maybe* they could share some region walking code,
> but I'm not sure there's going to be anything of significant size here.

We're already sharing a listener for type1 and spapr windows, spapr-v2
is a lot like type1, just different ioctls and therefore slightly
different setup.

> > > The more generally correct approach, which allows for more complex
> > > IOMMU arrangements and the possibility of new IOMMU types with pre-reg
> > > is:
> > >    1. Have the core implement both a mapping listener and a pre-reg
> > >       listener (optionally enabled by a per-iommu-type flag).
> > >       Basically the first one sees what *is* mapped, the second sees
> > >       what *could* be mapped.
> > 
> > This just sounds like different address spaces, address_space_memory vs
> > address_space_physical_memory
> 
> Um.. what?  I'm not even sure what you mean by
> address_space_physical_memory (I see no such thing in the source).

It doesn't exist, it's just my attempt to interpret "*is* mapped" vs
"*could* be mapped".

> The idea was that the (top level) pre-reg listener would spawn more
> listeners for any AS which could get (partially) mapped into the PCI
> addres space.
> 
> But.. I looked a bit closer and realised this scheme doesn't actually
> work.  IOMMU memory regions don't actually have a fixed target AS
> property (by which I mean the address space the IOMMU translates
> *into* rather than translates from - address_space_memory in most
> cases).  Instead any individual IOMMU mapping can point to a different
> AS supplied in the IOMMUTLB structure.

Why does that imply separate listeners?  We already do this in the
current listener.  This all sounds like things that are not unique to
spapr, only the actual ioctls are unique.

> > >    2. As now, the mapping listener listens on PCI address space, if
> > >       RAM blocks are added, immediately map them into the host IOMMU,
> > >       if guest IOMMU blocks appear register a notifier which will
> > >       mirror guest IOMMU mappings to the host IOMMU (this is what we
> > >       do now).
> > 
> > Right, this is done now, nothing new required.
> 
> Yes, I was just spelling that out for comparison with the other part.
> 
> > >    3. The pre-reg listener also listens on the PCI address space.  RAM
> > >       blocks added are pre-registered immediately.  But, if guest
> > >       IOMMU blocks are added, instead of registering a guest-iommu
> > >       notifier, we register another listener on the *target* AS of the
> > >       guest IOMMU, same callbacks as this one.  In practice that
> > >       target AS will almost always resolve to address_space_memory,
> > >       but this can at least in theory handle crazy guest setups with
> > >       multiple layers of IOMMU.
> > > 
> > >    4. Have to ensure that the pre-reg callbacks always happen before
> > >       the mapping calls.  For a system with an IOMMU backend which
> > >       requires pre-registration, but doesn't have a guest IOMMU, we
> > >       need to pre-reg, then host-iommu-map RAM blocks that appear in
> > >       PCI address space.
> 
> > Both of these just seem like details of the iommu-type (pre-reg)
> > specific parts of the listener, I'm not understanding what's
> > fundamentally different about it that we can't do it now, within a
> > single listener, nor do I see how it's all that different from x86.
> 
> So, we have two different address spaces to listen on (PCI and
> address_space_memory) so we need to different listener instances
> registered, clearly.

Yes, Alexey's current proposal has two listener instances, that's fine.

> Those two different listeners need to do different things.  1) maps
> memory blocks into VFIO and sets up a notifier to mirror guest IOMMU
> mappings into VFIO.  2) preregisters memory blocks, and it would be a
> bug if it ever saw a guest IOMMU.
> 
> So, what's left to share?

Well, 2) is exactly what type1 needs and 1) seems like what any
guest-visible IOMMU needs.  Look at the code we have now, there's some
small alignment testing, if iommu, else type1.  Maybe the code can be
clarified slightly by duplicating the testing into separate listeners,
but unless there's something fundamentally different between type1 and
spapr-v2, I think that that the vfio-iommu callouts should just be
modularized and the listener shared.  Otherwise we're just splitting the
code for convenience at the cost of code sharing and maintenance.
Thanks,

Alex