[PATCH v1 0/7] arm64/sve: Clean up KVM integration and optimise syscalls

[Mark Brown]

This patch series attempts to clarify the tracking of which set of
floating point registers we save on systems supporting SVE, particularly
with reference to KVM, and then uses the results of this clarification
to improve the performance of simple syscalls where we return directly
to userspace in cases where userspace is using SVE.

At present we track which register state is active by using the TIF_SVE
flag for the current task which also controls if userspace is able to
use SVE, this is reasonably straightforward if limiting but for KVM it
gets a bit hairy since we may have guest state loaded in registers. This
results in KVM modifying TIF_SVE for the VMM task while the guest is
running which doesn't entirely help make things easy to follow. To help
make things clearer the series changes things so that in addition to
TIF_SVE we explicitly track both the type of registers that are
currently saved in the task struct and the type of registers that we
should save when we do so. TIF_SVE then solely controls if userspace
can use SVE without trapping, it has no function for KVM guests and we
can remove the code for managing it from KVM.

The refactoring to add the separate tracking is initially done by adding
the new state together with checks that the state corresponds to
expectations when we look at it before subsequent patches make use of
the separated state, the goal being to both split out the more repetitive
bits of tha change and make it easier to debug any problems that might
arise.

With the state tracked separately we then start to optimise the
performance of syscalls when the process is using SVE. Currently every
syscall disables SVE for userspace which means that we need to trap to
EL1 again on the next SVE instruction, flush the SVE registers, and
reenable SVE for EL0, creating overhead for tasks that mix SVE and
syscalls. We build on the above refactoring to eliminate this overhead
for simple syscalls which return directly to userspace by:

 - Keeping SVE enabled.
 - Not flushing the SVE state.

The removal of flushing is within our currently documented ABI but is a
change in our effective ABI so a sysctl is provided to revert to current
behaviour in case of problems or to allow testing of userspace.  If we
don't want to do this I think we should tighten our ABI documentation,
previously Catalin had indicated that he didn't want to tighten it.

Mark Brown (7):
  KVM: arm64: Discard any SVE state when entering KVM guests
  arm64/fpsimd: Track the saved FPSIMD state type separately to TIF_SVE
  arm64/fpsimd: Have KVM explicitly say which FP registers to save
  arm64/fpsimd: Stop using TIF_SVE to manage register saving in KVM
  arm64/fpsimd: Load FP state based on recorded data type
  arm64/sve: Leave SVE enabled on syscall if we don't context switch
  arm64/sve: Don't zero non-FPSIMD register state on syscall by default

 arch/arm64/include/asm/fpsimd.h    |   4 +-
 arch/arm64/include/asm/kvm_host.h  |   1 +
 arch/arm64/include/asm/processor.h |   7 ++
 arch/arm64/kernel/fpsimd.c         | 136 +++++++++++++++++++++++------
 arch/arm64/kernel/process.c        |   2 +
 arch/arm64/kernel/ptrace.c         |   6 +-
 arch/arm64/kernel/signal.c         |   3 +
 arch/arm64/kernel/syscall.c        |  53 ++++++++---
 arch/arm64/kvm/fpsimd.c            |  16 ++--
 9 files changed, 180 insertions(+), 48 deletions(-)


base-commit: f2906aa863381afb0015a9eb7fefad885d4e5a56
-- 
2.30.2