[RFC,v3,00/27] KVM: arm64: Implement support for SME in non-protected guests

Message

Mark Brown Dec. 20, 2024, 4:46 p.m. UTC

Given the time of year and point in the release cycle this is an RFC
series, there's a few areas where I'm particularly expecting that people
might have feedback:

 - The userspace ABI, in particular:
  - The vector length used for the SVE registers, access to the SVE
    registers and access to ZA and (if available) ZT0 depending on
    the current state of PSTATE.{SM,ZA}.
  - The use of a single finalisation for both SVE and SME.

 - The addition of control for enabling fine grained traps in a similar
   manner to FGU but without the UNDEF, I'm not clear if this is desired
   at all and at present this requires symmetric read and write traps like
   FGU. That seemed like it might be desired from an implementation
   point of view but we already have one case where we enable an
   asymmetric trap (for ARM64_WORKAROUND_AMPERE_AC03_CPU_38) and it
   seems generally useful to enable asymmetrically.

There is some nested virtualisation support in the code but it is not
enabled or complete, this will be completed before the RFC tag is
removed. I am anticipating having a vastly better test environment soon
which will make this much easier to complete and there is no SME
specific ABI for nested virtualisation.

This series implements support for SME use in non-protected KVM guests.
Much of this is very similar to SVE, the main additional challenge that
SME presents is that it introduces a new vector length similar to the
SVE vector length and two new controls which change the registers seen
by guests:

 - PSTATE.ZA enables the ZA matrix register and, if SME2 is supported,
   the ZT0 LUT register.
 - PSTATE.SM enables streaming mode, a new floating point mode which
   uses the SVE register set with the separately configured SME vector
   length.  In streaming mode implementation of the FFR register is
   optional.

It is also permitted to build systems which support SME without SVE, in
this case when not in streaming mode no SVE registers or instructions
are available.  Further, there is no requirement that there be any
overlap in the set of vector lengths supported by SVE and SME in a
system, this is expected to be a common situation in practical systems.

Since there is a new vector length to configure we introduce a new
feature parallel to the existing SVE one with a new pseudo register for
the streaming mode vector length.  Due to the overlap with SVE caused by
streaming mode rather than finalising SME as a separate feature we use
the existing SVE finalisation to also finalise SME, a new define
KVM_ARM_VCPU_VEC is provided to help make user code clearer.  Finalising
SVE and SME separately would introduce complication with register access
since finalising SVE makes the SVE regsiters writeable by userspace and
doing multiple finalisations results in an error being reported.
Dealing with a state where the SVE registers are writeable due to one of
SVE or SME being finalised but may have their VL changed by the other
being finalised seems like needless complexity with minimal practical
utility, it seems clearer to just express directly that only one
finalisation can be done in the ABI.

Access to the floating point registers follows the architecture:

 - When both SVE and SME are present:
   - If PSTATE.SM == 0 the vector length used for the Z and P registers
     is the SVE vector length.
   - If PSTATE.SM == 1 the vector length used for the Z and P registers
     is the SME vector length.
 - If only SME is present:
   - If PSTATE.SM == 0 the Z and P registers are inaccessible and the
     floating point state accessed via the encodings for the V registers. 
   - If PSTATE.SM == 1 the vector length used for the Z and P registers
 - The SME specific ZA and ZT0 registers are only accessible if SVCR.ZA is 1.

The VMM must understand this, in particular when loading state SVCR
should be configured before other state.

There are a large number of subfeatures for SME, most of which only
offer additional instructions but some of which (SME2 and FA64) add
architectural state. These are configured via the ID registers as per
usual.

The new KVM_ARM_VCPU_VEC feature and ZA and ZT0 registers have not been
added to the get-reg-list selftest, the idea of supporting additional
features there without restructuring the program to generate all
possible feature combinations has been rejected.  I will post a separate
series which does that restructuring.

No support is present for protected guests, this is expected to be added
separately.

The series is based on Fuad's series:

  https://lore.kernel.org/r/[e-mail]/

It will need a rebase on:

  https://lore.kernel.org/r/[e-mail]

(as will Fuad's.)

Signed-off-by: Mark Brown <[e-mail]>
---
Changes in v3:
- Rebase onto v6.12-rc2.
- Link to v2: https://lore.kernel.org/r/[e-mail]

Changes in v2:
- Rebase onto v6.7-rc3.
- Configure subfeatures based on host system only.
- Complete nVHE support.
- There was some snafu with sending v1 out, it didn't make it to the
  lists but in case it hit people's inboxes I'm sending as v2.

---
Mark Brown (27):
      arm64/fpsimd: Update FA64 and ZT0 enables when loading SME state
      arm64/fpsimd: Decide to save ZT0 and streaming mode FFR at bind time
      arm64/fpsimd: Check enable bit for FA64 when saving EFI state
      arm64/fpsimd: Determine maximum virtualisable SME vector length
      KVM: arm64: Introduce non-UNDEF FGT control
      KVM: arm64: Pull ctxt_has_ helpers to start of sysreg-sr.h
      KVM: arm64: Convert cpacr_clear_set() to a static inline
      KVM: arm64: Move SVE state access macros after feature test macros
      KVM: arm64: Factor SVE guest exit handling out into a function
      KVM: arm64: Rename SVE finalization constants to be more general
      KVM: arm64: Document the KVM ABI for SME
      KVM: arm64: Define internal features for SME
      KVM: arm64: Rename sve_state_reg_region
      KVM: arm64: Store vector lengths in an array
      KVM: arm64: Implement SME vector length configuration
      KVM: arm64: Add definitions for SME control register
      KVM: arm64: Support TPIDR2_EL0
      KVM: arm64: Support SMIDR_EL1 for guests
      KVM: arm64: Support SME priority registers
      KVM: arm64: Provide assembly for SME state restore
      KVM: arm64: Support Z and P registers in streaming mode
      KVM: arm64: Expose SME specific state to userspace
      KVM: arm64: Context switch SME state for normal guests
      KVM: arm64: Handle SME exceptions
      KVM: arm64: Provide interface for configuring and enabling SME for guests
      KVM: arm64: selftests: Add SME system registers to get-reg-list
      KVM: arm64: selftests: Add SME to set_id_regs test

 Documentation/virt/kvm/api.rst                     | 117 ++++++---
 arch/arm64/include/asm/fpsimd.h                    |  22 ++
 arch/arm64/include/asm/kvm_emulate.h               |  37 ++-
 arch/arm64/include/asm/kvm_host.h                  | 135 ++++++++---
 arch/arm64/include/asm/kvm_hyp.h                   |   4 +-
 arch/arm64/include/asm/kvm_pkvm.h                  |   2 +-
 arch/arm64/include/asm/vncr_mapping.h              |   2 +
 arch/arm64/include/uapi/asm/kvm.h                  |  33 +++
 arch/arm64/kernel/cpufeature.c                     |   2 -
 arch/arm64/kernel/fpsimd.c                         |  86 ++++---
 arch/arm64/kvm/arm.c                               |  10 +
 arch/arm64/kvm/fpsimd.c                            | 156 +++++++-----
 arch/arm64/kvm/guest.c                             | 262 ++++++++++++++++++---
 arch/arm64/kvm/handle_exit.c                       |  14 ++
 arch/arm64/kvm/hyp/fpsimd.S                        |  16 ++
 arch/arm64/kvm/hyp/include/hyp/switch.h            | 104 ++++++--
 arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h         |  47 ++--
 arch/arm64/kvm/hyp/nvhe/hyp-main.c                 |  17 +-
 arch/arm64/kvm/hyp/nvhe/pkvm.c                     |   4 +-
 arch/arm64/kvm/hyp/nvhe/switch.c                   |  11 +-
 arch/arm64/kvm/hyp/vhe/switch.c                    |  21 +-
 arch/arm64/kvm/reset.c                             | 154 +++++++++---
 arch/arm64/kvm/sys_regs.c                          | 118 +++++++++-
 include/uapi/linux/kvm.h                           |   1 +
 tools/testing/selftests/kvm/aarch64/get-reg-list.c |  32 ++-
 tools/testing/selftests/kvm/aarch64/set_id_regs.c  |  29 ++-
 26 files changed, 1117 insertions(+), 319 deletions(-)
---
base-commit: e32a80927434907f973f38a88cd19d7e51991d24
change-id: 20230301-kvm-arm64-sme-06a1246d3636
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Best regards,