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

[David Gibson]

On Fri, Jul 17, 2015 at 05:13:37PM +1000, Alexey Kardashevskiy wrote:
> On 07/16/2015 03:11 PM, 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
> >
> >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.
> >
> >    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).
> >
> >    3. The pre-reg listener also listens on the PCI address space.  RAM
> >       blocks added are pre-registered immediately.
> 
> 
> PCI address space listeners won't be notified about RAM blocks on sPAPR.

Sure they will - if any RAM blocks were mapped directly into PCI
address space, the listener would be notified.  It's just that no RAM
blocks are directly mapped into PCI space, only partially mapped in
via IOMMU blocks.

But the idea is this scheme could handle a platform that has both a
"bypass" DMA window which maps directly onto a block of ram and an
IOMMU controlled DMA window.  Or one which could have either setup
depending on circumstances (which is probably true of BenH's "powernv"
machine type).

> >But, if guest
> >       IOMMU blocks are added, instead of registering a guest-iommu
> >       notifier,
> 
> "guest-iommu notifier" is the one called via memory_region_notify_iommu()
> from H_PUT_TCE? "Instead" implies dropping it, how this can work?

Because the other listener - the mapping listener at (2) handles that
part.  The pre-reg listener doesn't.

But as noted in by other mail this whole scheme doesn't work without a
way to discover an IOMMU region's target AS in advance, which doesn't
currently exist.

> >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.

-- 
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

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