Re: [RFC PATCH v2 00/13] SMMUv3 nested translation support

[Eric Auger]

Hi Mostafa,

On 4/8/24 16:08, Mostafa Saleh wrote:
> Currently, QEMU supports emulating either stage-1 or stage-2 SMMUs
> but not nested instances.
> This patch series adds support for nested translation in SMMUv3,
> this is controlled by property “arm-smmuv3.stage=nested”, and
> advertised to guests as (IDR0.S1P == 1 && IDR0.S2P == 2)
>
> Main changes(architecture):
> ============================
> 1) CDs are considered IPA and translated with stage-2.
> 2) TTBx and tables for stage-1 are considered IPA and translated
>    with stage-2.
> 3) Translate the IPA address with stage-2.
>
> TLBs:
> ======
> TLBs are the most tricky part.
>
> 1) General design
>    Unified(Combined) design is used, where entries with ASID=-1 are
>    IPAs(cached from stage-2 config)
>
>    TLBs are also modified to cache 2 permissions, a new permission added
>    "parent_perm."
>
>    For non-nested configuration, perm == parent_perm and nothing
>    changes. This is used to know which stage to use in case there is
>    a permission fault from a TLB entry.
>
> 2) Caching in TLB
>    Stage-1 and stage-2 are inserted in the TLB as is.
>    For nested translation, both entries are combined into one TLB
>    entry. The size (level and granule) are chosen from the smallest entries.
>    That means that a stage-1 translation can be cached with sage-2
>    granule in key, this is take into account lookup.
is that a correct understanding that with the current implementation, in
nested mode, you end up with combined S1 + S2 entries (IOVA -> PA) and
S2 entries (IPA -> PA)?
Out of cusiosity, how did you end up with that choice? Have you made
some perf assessment compared to separate S1 and S2 entries? I guess it
is a complex topic and choice.

Thanks

Eric
>
> 3) TLB Lookup
>    TLB lookup already uses ASID in key, so it can distinguish between
>    stage-1 and stage-2.
>    And as mentioned above, the granule for stage-1 can be different,
>    If stage-1 lookup failed, we try again with the stage-2 granule.
>
> 4) TLB invalidation
>    - Address invalidation is split, for IOVA(CMD_TLBI_NH_VA
>      /CMD_TLBI_NH_VAA) and IPA(CMD_TLBI_S2_IPA) based on ASID value
>    - CMD_TLBI_NH_ASID/CMD_TLBI_NH_ALL: Consider VMID if stage-2 is
>      supported, and invalidate stage-1 only by VMIDs
>
> As far as I understand, this is compliant with the ARM architecture:
> - ARM ARM DDI 0487J.a: RLGSCG, RTVTYQ, RGNJPZ
> - ARM IHI 0070F.b: 16.2 Caching
>
> An alternative approach would be to instantiate 2 TLBs, one per each
> stage. I haven’t investigated that.
>
> Others
> =======
> - Advertise SMMUv3.2-S2FWB, it is NOP for QEMU as it doesn’t support
>   attributes.
>
> - OAS: A typical setup with nesting is to share CPU stage-2 with the
>   SMMU, and according to the user manual, SMMU OAS must match the
>   system physical address.
>
>   This was discussed before in
>   https://lore.kernel.org/all/20230226220650.1480786-11-smostafa@google.com/
>   The implementation here, follows the discussion, where migration is
>   added and oas is set up from the board (virt). However, the OAS is
>   chosen based on the CPU PARANGE as there is no fixed one.
>
> - For nested configuration, IOVA notifier only notifies for stage-1
>   invalidations (as far as I understand this is the intended
>   behaviour as it notifies for IOVA)
>
> - Stop ignoring VMID for stage-1 if stage-2 is also supported.
>
>
> Future improvements:
> =====================
> 1) One small improvement, that I don’t think it’s worth the extra
>    complexity, is in case of Stage-1 TLB miss for nested translation,
>    we can do stage-1 walk and lookup for stage-2 TLBs, instead of
>    doing the full walk.
>
> 2) Patch 0006 (hw/arm/smmuv3: Translate CD and TT using stage-2 table)
>    introduces a macro to use functions that rely on cfg for stage-2,
>    I don’t like it. However, I didn’t find a simple way around it,
>    either we change many functions to have a separate stage argument,
>    or add another arg in config, which is probably more code.
>
> Testing
> ========
> 1) IOMMUFD + VFIO
>    Kernel: 
> https://lore.kernel.org/all/cover.1683688960.git.nicolinc@nvidia.com/
>    VMM: 
> https://qemu-devel.nongnu.narkive.com/o815DqpI/rfc-v5-0-8-arm-smmuv3-emulation-support
>
>    By assigning 
> “virtio-net-pci,netdev=net0,disable-legacy=on,iommu_platform=on,ats=on”,
>    to a guest VM (on top of QEMU guest) with VIFO and IOMMUFD.
>
> 2) Work in progress prototype I am hacking on for nesting on KVM
>    (this is nowhere near complete, and misses many stuff but it
>    doesn't require VMs/VFIO) also with virtio-net-pci and git
>    cloning a bunch of stuff and also observing traces.
>    
> https://android-kvm.googlesource.com/linux/+log/refs/heads/smostafa/android15-6.6-smmu-nesting-wip
>
> I also modified the Linux driver to test with mixed granules/levels.
>
> hw/arm/smmuv3: Split smmuv3_translate() better viewed with --color-moved
>
> Changes in v2:
> v1: 
> https://lore.kernel.org/qemu-devel/20240325101442.1306300-1-smostafa@google.com/
> - Collected Eric Rbs
> - Rework TLB to rely on VMID/ASID instead of an extra key.
> - Fixed TLB issue with large stage-1 reported by Julian.
> - Cap the OAS to 48 bits as PTW doesn’t support 52 bits.
> - Fix ASID/VMID representation in some contexts as 16 bits while
>   they can be -1
> - Increase visibility in trace points
>
>
> Mostafa Saleh (13):
>   hw/arm/smmu: Use enum for SMMU stage
>   hw/arm/smmu: Split smmuv3_translate()
>   hw/arm/smmu: Consolidate ASID and VMID types
>   hw/arm/smmuv3: Translate CD and TT using stage-2 table
>   hw/arm/smmu-common: Support nested translation
>   hw/arm/smmu: Support nesting in smmuv3_range_inval()
>   hw/arm/smmu: Support nesting in the rest of commands
>   hw/arm/smmuv3: Support nested SMMUs in smmuv3_notify_iova()
>   hw/arm/smmuv3: Support and advertise nesting
>   hw/arm/smmuv3: Advertise S2FWB
>   hw/arm/smmu: Refactor SMMU OAS
>   hw/arm/smmuv3: Add property for OAS
>   hw/arm/virt: Set SMMU OAS based on CPU PARANGE
>
>  hw/arm/smmu-common.c         | 306 +++++++++++++++++++++----
>  hw/arm/smmuv3-internal.h     |  16 +-
>  hw/arm/smmuv3.c              | 418 ++++++++++++++++++++++-------------
>  hw/arm/trace-events          |  18 +-
>  hw/arm/virt.c                |  14 +-
>  include/hw/arm/smmu-common.h |  57 ++++-
>  include/hw/arm/smmuv3.h      |   1 +
>  target/arm/cpu.h             |   2 +
>  target/arm/cpu64.c           |   5 +
>  9 files changed, 608 insertions(+), 229 deletions(-)
>