Re: [PATCH v11 00/40] arm64/sme: Initial support for the Scalable Matrix Extension

[Shuah Khan <skhan@linuxfoundation.org>]

On 2/7/22 8:20 AM, Mark Brown wrote:
> This series provides initial support for the ARMv9 Scalable Matrix
> Extension (SME).  SME takes the approach used for vectors in SVE and
> extends this to provide architectural support for matrix operations.  A
> more detailed overview can be found in [1].
> 
> For the kernel SME can be thought of as a series of features which are
> intended to be used together by applications but operate mostly
> orthogonally:
> 
>   - The ZA matrix register.
>   - Streaming mode, in which ZA can be accessed and a subset of SVE
>     features are available.
>   - A second vector length, used for streaming mode SVE and ZA and
>     controlled using a similar interface to that for SVE.
>   - TPIDR2, a new userspace controllable system register intended for use
>     by the C library for storing context related to the ZA ABI.
> 
> A substantial part of the series is dedicated to refactoring the
> existing SVE support so that we don't need to duplicate code for
> handling vector lengths and the SVE registers, this involves creating an
> array of vector types and making the users take the vector type as a
> parameter.  I'm not 100% happy with this but wasn't able to come up with
> anything better, duplicating code definitely felt like a bad idea so
> this felt like the least bad thing.  If this approach makes sense to
> people it might make sense to split this off into a separate series
> and/or merge it while the rest is pending review to try to make things a
> little more digestable, the series is very large so it'd probably make
> things easier to digest if some of the preparatory refactoring could be
> merged before the rest is ready.
> 
> One feature of the architecture of particular note is that switching
> to and from streaming mode may change the size of and invalidate the
> contents of the SVE registers, and when in streaming mode the FFR is not
> accessible.  This complicates aspects of the ABI like signal handling
> and ptrace.
> 
> This initial implementation is mainly intended to get the ABI in place,
> there are several areas which will be worked on going forwards - some of
> these will be blockers, others could be handled in followup serieses:
> 
>   - SME is currently not supported for KVM guests, this will be done as a
>     followup series.  A host system can use SME and run KVM guests but
>     SME is not available in the guests.
>   - The KVM host support is done in a very simplistic way, were anyone to
>     attempt to use it in production there would be performance impacts on
>     hosts with SME support. As part of this we also add enumeration of
>     fine grained traps.
>   - There is not currently ptrace or signal support TPIDR2, this will be
>     done as a followup series.
>   - No support is currently provided for scheduler control of SME or SME
>     applications, given the size of the SME register state the context
>     switch overhead may be noticable so this may be needed especially for
>     real time applications.  Similar concerns already exist for larger
>     SVE vector lengths but are amplified for SME, particularly as the
>     vector length increases.
>   - There has been no work on optimising the performance of anything the
>     kernel does.
> 
> It is not expected that any systems will be encountered that support SME
> but not SVE, SME is an ARMv9 feature and SVE is mandatory for ARMv9.
> The code attempts to handle any such systems that are encountered but
> this hasn't been tested extensively.
> 
> v11:
>   - Rebase onto v5.17-rc3.
>   - Provide a sme-inst.h to collect manual encodings in kselftest.
> v10:
>   - Actually do the rebase of fixups from the previous version into
>     relevant patches.
> v9:
>   - Remove defensive programming around IS_ENABLED() and FGT in KVM code.
>   - Fix naming of TPIDR2 FGT register bit.
>   - Add patches making handling of floating point register bits more
>     consistent (also sent as separate series).
>   - Drop now unused enumeration of fine grained traps.
> v8:
>   - Rebase onto v5.17-rc1.
>   - Support interoperation with KVM, SME is disabled for KVM guests with
>     minimal handling for cleaning up SME state when entering and leaving
>     the guest.
>   - Document and implement that signal handlers are invoked with ZA and
>     streaming mode disabled.
>   - Use the RDSVL instruction introduced in EAC2 of the architecture to
>     obtain the streaming mode vector length during enumeration, ZA state
>     loading/saving and in test programs.
>   - Store a pointer to SVCR in fpsimd_last_state and use it in fpsimd_save()
>     for interoperation with KVM.
>   - Add a test case sme_trap_no_sm checking that we generate a SIGILL
>     when using an instruction that requires streaming mode without
>     enabling it.
>   - Add basic ZA context form validation to testcases helper library.
>   - Move signal tests over to validating streaming VL from ZA information.
>   - Pulled in patch removing ARRAY_SIZE() so that kselftest builds
>     cleanly and to avoid trivial conflicts.
> v7:
>   - Rebase onto v5.16-rc3.
>   - Reduce indentation when supporting custom triggers for signal tests
>     as suggested by Catalin.
>   - Change to specifying a width for all CPU features rather than adding
>     single bit specific infrastructure.
>   - Don't require zeroing of non-shared SVE state during syscalls.
> v6:
>   - Rebase onto v5.16-rc1.
>   - Return to disabling TIF_SVE on kernel entry even if we have SME
>     state, this avoids the need for KVM to handle the case where TIF_SVE
>     is set on guest entry.
>   - Add syscall-abi.h to SME updates to syscall-abi, mistakenly omitted
>     from commit.
> v5:
>   - Rebase onto currently merged SVE and kselftest patches.
>   - Add support for the FA64 option, introduced in the recently published
>     EAC1 update to the specification.
>   - Pull in test program for the syscall ABI previously sent separately
>     with some revisions and add coverage for the SME ABI.
>   - Fix checking for options with 1 bit fields in ID_AA64SMFR0_EL1.
>   - Minor fixes and clarifications to the ABI documentation.
> v4:
>   - Rebase onto merged patches.
>   - Remove an uneeded NULL check in vec_proc_do_default_vl().
>   - Include patch to factor out utility routines in kselftests written in
>     assembler.
>   - Specify -ffreestanding when building TPIDR2 test.
> v3:
>   - Skip FFR rather than predicate registers in sve_flush_live().
>   - Don't assume a bool is all zeros in sve_flush_live() as per AAPCS.
>   - Don't redundantly specify a zero index when clearing FFR.
> v2:
>   - Fix several issues with !SME and !SVE configurations.
>   - Preserve TPIDR2 when creating a new thread/process unless
>     CLONE_SETTLS is set.
>   - Report traps due to using features in an invalid mode as SIGILL.
>   - Spell out streaming mode behaviour in SVE ABI documentation more
>     directly.
>   - Document TPIDR2 in the ABI document.
>   - Use SMSTART and SMSTOP rather than read/modify/write sequences.
>   - Rework logic for exiting streaming mode on syscall.
>   - Don't needlessly initialise SVCR on access trap.
>   - Always restore SME VL for userspace if SME traps are disabled.
>   - Only yield to encourage preemption every 128 iterations in za-test,
>     otherwise do a getpid(), and validate SVCR after syscall.
>   - Leave streaming mode disabled except when reading the vector length
>     in za-test, and disable ZA after detecting a mismatch.
>   - Add SME support to vlset.
>   - Clarifications and typo fixes in comments.
>   - Move sme_alloc() forward declaration back a patch.
> 
> [1] https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/scalable-matrix-extension-armv9-a-architecture
> 
> Mark Brown (40):
>    arm64: Define CPACR_EL1_FPEN similarly to other floating point
>      controls
>    arm64: Always use individual bits in CPACR floating point enables
>    arm64: cpufeature: Always specify and use a field width for
>      capabilities
>    kselftest/arm64: Remove local ARRAY_SIZE() definitions
>    kselftest/arm64: signal: Allow tests to be incompatible with features
>    arm64/sme: Provide ABI documentation for SME
>    arm64/sme: System register and exception syndrome definitions
>    arm64/sme: Manually encode SME instructions
>    arm64/sme: Early CPU setup for SME
>    arm64/sme: Basic enumeration support
>    arm64/sme: Identify supported SME vector lengths at boot
>    arm64/sme: Implement sysctl to set the default vector length
>    arm64/sme: Implement vector length configuration prctl()s
>    arm64/sme: Implement support for TPIDR2
>    arm64/sme: Implement SVCR context switching
>    arm64/sme: Implement streaming SVE context switching
>    arm64/sme: Implement ZA context switching
>    arm64/sme: Implement traps and syscall handling for SME
>    arm64/sme: Disable ZA and streaming mode when handling signals
>    arm64/sme: Implement streaming SVE signal handling
>    arm64/sme: Implement ZA signal handling
>    arm64/sme: Implement ptrace support for streaming mode SVE registers
>    arm64/sme: Add ptrace support for ZA
>    arm64/sme: Disable streaming mode and ZA when flushing CPU state
>    arm64/sme: Save and restore streaming mode over EFI runtime calls
>    KVM: arm64: Hide SME system registers from guests
>    KVM: arm64: Trap SME usage in guest
>    KVM: arm64: Handle SME host state when running guests
>    arm64/sme: Provide Kconfig for SME
>    kselftest/arm64: Add manual encodings for SME instructions
>    kselftest/arm64: sme: Add SME support to vlset
>    kselftest/arm64: Add tests for TPIDR2
>    kselftest/arm64: Extend vector configuration API tests to cover SME
>    kselftest/arm64: sme: Provide streaming mode SVE stress test
>    kselftest/arm64: signal: Handle ZA signal context in core code
>    kselftest/arm64: Add stress test for SME ZA context switching
>    kselftest/arm64: signal: Add SME signal handling tests
>    kselftest/arm64: Add streaming SVE to SVE ptrace tests
>    kselftest/arm64: Add coverage for the ZA ptrace interface
>    kselftest/arm64: Add SME support to syscall ABI test
> 
>   Documentation/arm64/elf_hwcaps.rst            |  33 +
>   Documentation/arm64/index.rst                 |   1 +
>   Documentation/arm64/sme.rst                   | 432 +++++++++++++
>   Documentation/arm64/sve.rst                   |  70 ++-
>   arch/arm64/Kconfig                            |  11 +
>   arch/arm64/include/asm/cpu.h                  |   4 +
>   arch/arm64/include/asm/cpufeature.h           |  25 +
>   arch/arm64/include/asm/el2_setup.h            |  64 +-
>   arch/arm64/include/asm/esr.h                  |  13 +-
>   arch/arm64/include/asm/exception.h            |   1 +
>   arch/arm64/include/asm/fpsimd.h               | 110 +++-
>   arch/arm64/include/asm/fpsimdmacros.h         |  86 +++
>   arch/arm64/include/asm/hwcap.h                |   8 +
>   arch/arm64/include/asm/kvm_arm.h              |   5 +-
>   arch/arm64/include/asm/kvm_host.h             |   4 +
>   arch/arm64/include/asm/processor.h            |  18 +-
>   arch/arm64/include/asm/sysreg.h               |  67 +-
>   arch/arm64/include/asm/thread_info.h          |   2 +
>   arch/arm64/include/uapi/asm/hwcap.h           |   8 +
>   arch/arm64/include/uapi/asm/ptrace.h          |  69 ++-
>   arch/arm64/include/uapi/asm/sigcontext.h      |  55 +-
>   arch/arm64/kernel/cpufeature.c                | 273 ++++++--
>   arch/arm64/kernel/cpuinfo.c                   |  13 +
>   arch/arm64/kernel/entry-common.c              |  11 +
>   arch/arm64/kernel/entry-fpsimd.S              |  36 ++
>   arch/arm64/kernel/fpsimd.c                    | 585 ++++++++++++++++--
>   arch/arm64/kernel/process.c                   |  28 +-
>   arch/arm64/kernel/ptrace.c                    | 356 +++++++++--
>   arch/arm64/kernel/signal.c                    | 194 +++++-
>   arch/arm64/kernel/syscall.c                   |  34 +-
>   arch/arm64/kernel/traps.c                     |   1 +
>   arch/arm64/kvm/fpsimd.c                       |  43 +-
>   arch/arm64/kvm/hyp/include/hyp/switch.h       |   4 +-
>   arch/arm64/kvm/hyp/nvhe/switch.c              |  30 +
>   arch/arm64/kvm/hyp/vhe/switch.c               |  15 +-
>   arch/arm64/kvm/sys_regs.c                     |   9 +-
>   arch/arm64/tools/cpucaps                      |   2 +
>   include/uapi/linux/elf.h                      |   2 +
>   include/uapi/linux/prctl.h                    |   9 +
>   kernel/sys.c                                  |  12 +
>   tools/testing/selftests/arm64/abi/.gitignore  |   1 +
>   tools/testing/selftests/arm64/abi/Makefile    |   9 +-
>   .../selftests/arm64/abi/syscall-abi-asm.S     |  69 ++-
>   .../testing/selftests/arm64/abi/syscall-abi.c | 205 +++++-
>   .../testing/selftests/arm64/abi/syscall-abi.h |  15 +
>   tools/testing/selftests/arm64/abi/tpidr2.c    | 298 +++++++++
>   tools/testing/selftests/arm64/fp/.gitignore   |   4 +
>   tools/testing/selftests/arm64/fp/Makefile     |  12 +-
>   tools/testing/selftests/arm64/fp/rdvl-sme.c   |  14 +
>   tools/testing/selftests/arm64/fp/rdvl.S       |  10 +
>   tools/testing/selftests/arm64/fp/rdvl.h       |   1 +
>   tools/testing/selftests/arm64/fp/sme-inst.h   |  51 ++
>   tools/testing/selftests/arm64/fp/ssve-stress  |  59 ++
>   tools/testing/selftests/arm64/fp/sve-ptrace.c |  13 +-
>   tools/testing/selftests/arm64/fp/sve-test.S   |  20 +
>   tools/testing/selftests/arm64/fp/vec-syscfg.c |  10 +
>   tools/testing/selftests/arm64/fp/vlset.c      |  10 +-
>   tools/testing/selftests/arm64/fp/za-ptrace.c  | 354 +++++++++++
>   tools/testing/selftests/arm64/fp/za-stress    |  59 ++
>   tools/testing/selftests/arm64/fp/za-test.S    | 388 ++++++++++++
>   .../testing/selftests/arm64/signal/.gitignore |   2 +
>   .../selftests/arm64/signal/test_signals.h     |   5 +
>   .../arm64/signal/test_signals_utils.c         |  40 +-
>   .../arm64/signal/test_signals_utils.h         |   2 +
>   .../testcases/fake_sigreturn_sme_change_vl.c  |  92 +++
>   .../arm64/signal/testcases/sme_trap_no_sm.c   |  38 ++
>   .../signal/testcases/sme_trap_non_streaming.c |  45 ++
>   .../arm64/signal/testcases/sme_trap_za.c      |  36 ++
>   .../selftests/arm64/signal/testcases/sme_vl.c |  68 ++
>   .../arm64/signal/testcases/ssve_regs.c        | 129 ++++
>   .../arm64/signal/testcases/testcases.c        |  36 ++
>   .../arm64/signal/testcases/testcases.h        |   3 +-
>   72 files changed, 4590 insertions(+), 251 deletions(-)
>   create mode 100644 Documentation/arm64/sme.rst
>   create mode 100644 tools/testing/selftests/arm64/abi/syscall-abi.h
>   create mode 100644 tools/testing/selftests/arm64/abi/tpidr2.c
>   create mode 100644 tools/testing/selftests/arm64/fp/rdvl-sme.c
>   create mode 100644 tools/testing/selftests/arm64/fp/sme-inst.h
>   create mode 100644 tools/testing/selftests/arm64/fp/ssve-stress
>   create mode 100644 tools/testing/selftests/arm64/fp/za-ptrace.c
>   create mode 100644 tools/testing/selftests/arm64/fp/za-stress
>   create mode 100644 tools/testing/selftests/arm64/fp/za-test.S
>   create mode 100644 tools/testing/selftests/arm64/signal/testcases/fake_sigreturn_sme_change_vl.c
>   create mode 100644 tools/testing/selftests/arm64/signal/testcases/sme_trap_no_sm.c
>   create mode 100644 tools/testing/selftests/arm64/signal/testcases/sme_trap_non_streaming.c
>   create mode 100644 tools/testing/selftests/arm64/signal/testcases/sme_trap_za.c
>   create mode 100644 tools/testing/selftests/arm64/signal/testcases/sme_vl.c
>   create mode 100644 tools/testing/selftests/arm64/signal/testcases/ssve_regs.c
> 
> 
> base-commit: dfd42facf1e4ada021b939b4e19c935dcdd55566
> 

Mark,

Kselftest patches look good to me with your responses to my comments
on individual patches. In the interest of not generating more email
traffic - responding to all here. I already added Reviewed-by to the
patches.

Just the doc patch - SPDX needs addressing.

thanks,
-- Shuah

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