Re: [PATCH v4] x86: load FPU registers on return to userland

[Wanpeng Li]

On Thu, 8 Nov 2018 at 03:55, Sebastian Andrzej Siewior
<bigeasy@xxxxxxxxxxxxx> wrote:
>
> This is a refurbished series originally started by by Rik van Riel. The
> goal is load the FPU registers on return to userland and not on every
> context switch. By this optimisation we can:
> - avoid loading the registers if the task stays in kernel and does
>   not return to userland
> - make kernel_fpu_begin() cheaper: it only saves the registers on the
>   first invocation. The second invocation does not need save them again.
>

Do you have any performance data?

Regards,
Wanpeng Li

> To access the FPU registers in kernel we need:
> - disable preemption to avoid that the scheduler switches tasks. By
>   doing so it would set TIF_NEED_FPU_LOAD and the FPU registers would be
>   not valid.
> - disable BH because the softirq might use kernel_fpu_begin() and then
>   set TIF_NEED_FPU_LOAD instead loading the FPU registers on completion.
>
> v3âv4:
> It has been suggested to remove the `initialized' member of the struct
> fpu because it should not required be needed with lazy-FPU-restore and
> would make the review easier. This is the first part of the series, the
> second is basically the rebase of the v3 queue. As a result, the
> diffstat became negative (which wasn't the case in previous version) :)
> I tried to incorporate all the review comments that came up, some of
> them were "outdated" after the removal of the `initialized' member. I'm
> sorry should I missed any.
>
> v1âv3:
> v2 was never posted. I followed the idea to completely decouple PKRU
> from xstate. This didn't quite work and made a few things complicated.
> One obvious required fixup is copy_fpstate_to_sigframe() where the PKRU
> state needs to be fiddled into xstate. This required another
> xfeatures_mask so that the sanity checks were performed and
> xstate_offsets would be computed. Additionally ptrace also reads/sets
> xstate in order to get/set the register and PKRU is one of them. So this
> would need some fiddle, too.
> In v3 I dropped that decouple idea. I also learned that the wrpkru
> instruction is not privileged and so caching it in kernel does not work.
> Instead I keep PKRU in xstate area and load it at context switch time
> while the remaining registers are deferred (until return to userland).
> The offset of PKRU within xstate is enumerated at boot time so why not
> use it.
>
> Rik van Riel (5):
>   x86/fpu: Add (__)make_fpregs_active helpers
>   x86/fpu: Eager switch PKRU state
>   x86/fpu: Always store the registers in copy_fpstate_to_sigframe()
>   x86/fpu: Prepare copy_fpstate_to_sigframe() for TIF_NEED_FPU_LOAD
>   x86/fpu: Defer FPU state load until return to userspace
>
> Sebastian Andrzej Siewior (18):
>   x86/fpu: Use ULL for shift in xfeature_uncompacted_offset()
>   x86/fpu: Remove fpu->initialized usage in __fpu__restore_sig()
>   x86/fpu: Remove fpu__restore()
>   x86/entry/32: Remove asm/math_emu.h include
>   x86/fpu: Remove preempt_disable() in fpu__clear()
>   x86/fpu: Always init the `state' in fpu__clear()
>   x86/fpu: Remove fpu->initialized usage in copy_fpstate_to_sigframe()
>   x86/fpu: Remove fpu->initialized
>   x86/fpu: Remove user_fpu_begin()
>   x86/entry: Remove _TIF_ALLWORK_MASK
>   x86/fpu: Make __raw_xsave_addr() use feature number instead of mask
>   x86/fpu: Make get_xsave_field_ptr() and get_xsave_addr() use feature
>     number instead of mask
>   x86/pkeys: Make init_pkru_value static
>   x86/fpu: Only write PKRU if it is different from current
>   x86/pkeys: Don't check if PKRU is zero before writting it
>   x86/entry: Add TIF_NEED_FPU_LOAD
>   x86/fpu: Update xstate's PKRU value on write_pkru()
>   x86/fpu: Don't restore the FPU state directly from userland in
>     __fpu__restore_sig()
>
>  Documentation/preempt-locking.txt    |   1 -
>  arch/x86/entry/common.c              |   8 ++
>  arch/x86/ia32/ia32_signal.c          |  17 +--
>  arch/x86/include/asm/fpu/api.h       |  25 ++++
>  arch/x86/include/asm/fpu/internal.h  | 149 ++++++----------------
>  arch/x86/include/asm/fpu/signal.h    |   2 +-
>  arch/x86/include/asm/fpu/types.h     |   9 --
>  arch/x86/include/asm/fpu/xstate.h    |   6 +-
>  arch/x86/include/asm/pgtable.h       |  19 ++-
>  arch/x86/include/asm/special_insns.h |  13 +-
>  arch/x86/include/asm/thread_info.h   |  10 +-
>  arch/x86/include/asm/trace/fpu.h     |   8 +-
>  arch/x86/kernel/fpu/core.c           | 181 +++++++++++++--------------
>  arch/x86/kernel/fpu/init.c           |   2 -
>  arch/x86/kernel/fpu/regset.c         |  24 +---
>  arch/x86/kernel/fpu/signal.c         | 133 +++++++++-----------
>  arch/x86/kernel/fpu/xstate.c         |  45 ++++---
>  arch/x86/kernel/process.c            |   2 +-
>  arch/x86/kernel/process_32.c         |  14 +--
>  arch/x86/kernel/process_64.c         |  11 +-
>  arch/x86/kernel/signal.c             |  17 ++-
>  arch/x86/kernel/traps.c              |   2 +-
>  arch/x86/kvm/x86.c                   |  47 ++++---
>  arch/x86/math-emu/fpu_entry.c        |   3 -
>  arch/x86/mm/mpx.c                    |   6 +-
>  arch/x86/mm/pkeys.c                  |  15 +--
>  26 files changed, 343 insertions(+), 426 deletions(-)
>
>  git://git.kernel.org/pub/scm/linux/kernel/git/bigeasy/staging.git x86_fpu_rtu_v4
>
> Sebastian
>