Re: [PATCH v9 0/2] Renovate memcpy_mcsafe with copy_mc_to_{user, kernel}

[Dan Williams]

On Wed, Sep 23, 2020 at 10:00 AM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> Changes since v8 [1]:
> - Rebase on v5.9-rc6
>
> - Fix a performance regression in the x86 copy_mc_to_user()
>   implementation that was duplicating copies in the "fragile" case.
>
> - Refreshed the cover letter.
>
> [1]: http://lore.kernel.org/r/159630255616.3143511.18110575960499749012.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>

Given that Linus was the primary source of review feedback on these
patches and a version of them have been soaking in -next with only a
minor conflict report with vfs.git for the entirety of the v5.9-rc
cycle (left there inadvertently while I was on leave), any concerns
with me sending this to Linus directly during the merge window?

>
> The motivations to go rework memcpy_mcsafe() are that the benefit of
> doing slow and careful copies is obviated on newer CPUs, and that the
> current opt-in list of cpus to instrument recovery is broken relative to
> those cpus.  There is no need to keep an opt-in list up to date on an
> ongoing basis if pmem/dax operations are instrumented for recovery by
> default. With recovery enabled by default the old "mcsafe_key" opt-in to
> careful copying can be made a "fragile" opt-out. Where the "fragile"
> list takes steps to not consume poison across cachelines.
>
> The discussion with Linus made clear that the current "_mcsafe" suffix
> was imprecise to a fault. The operations that are needed by pmem/dax are
> to copy from a source address that might throw #MC to a destination that
> may write-fault, if it is a user page. So copy_to_user_mcsafe() becomes
> copy_mc_to_user() to indicate the separate precautions taken on source
> and destination. copy_mc_to_kernel() is introduced as a version that
> does not expect write-faults on the destination, but is still prepared
> to abort with an error code upon taking #MC.
>
> These patches have received a kbuild-robot build success notification
> across 114 configs, the rebase to v5.9-rc6 did not encounter any
> conflicts, and the merge with tip/master is conflict-free.
>
> ---
>
> Dan Williams (2):
>       x86, powerpc: Rename memcpy_mcsafe() to copy_mc_to_{user,kernel}()
>       x86/copy_mc: Introduce copy_mc_generic()
>
>
>  arch/powerpc/Kconfig                               |    2
>  arch/powerpc/include/asm/string.h                  |    2
>  arch/powerpc/include/asm/uaccess.h                 |   40 +++--
>  arch/powerpc/lib/Makefile                          |    2
>  arch/powerpc/lib/copy_mc_64.S                      |    4
>  arch/x86/Kconfig                                   |    2
>  arch/x86/Kconfig.debug                             |    2
>  arch/x86/include/asm/copy_mc_test.h                |   75 +++++++++
>  arch/x86/include/asm/mcsafe_test.h                 |   75 ---------
>  arch/x86/include/asm/string_64.h                   |   32 ----
>  arch/x86/include/asm/uaccess.h                     |   21 +++
>  arch/x86/include/asm/uaccess_64.h                  |   20 --
>  arch/x86/kernel/cpu/mce/core.c                     |    8 -
>  arch/x86/kernel/quirks.c                           |    9 -
>  arch/x86/lib/Makefile                              |    1
>  arch/x86/lib/copy_mc.c                             |   65 ++++++++
>  arch/x86/lib/copy_mc_64.S                          |  165 ++++++++++++++++++++
>  arch/x86/lib/memcpy_64.S                           |  115 --------------
>  arch/x86/lib/usercopy_64.c                         |   21 ---
>  drivers/md/dm-writecache.c                         |   15 +-
>  drivers/nvdimm/claim.c                             |    2
>  drivers/nvdimm/pmem.c                              |    6 -
>  include/linux/string.h                             |    9 -
>  include/linux/uaccess.h                            |    9 +
>  include/linux/uio.h                                |   10 +
>  lib/Kconfig                                        |    7 +
>  lib/iov_iter.c                                     |   43 +++--
>  tools/arch/x86/include/asm/mcsafe_test.h           |   13 --
>  tools/arch/x86/lib/memcpy_64.S                     |  115 --------------
>  tools/objtool/check.c                              |    5 -
>  tools/perf/bench/Build                             |    1
>  tools/perf/bench/mem-memcpy-x86-64-lib.c           |   24 ---
>  tools/testing/nvdimm/test/nfit.c                   |   48 +++---
>  .../testing/selftests/powerpc/copyloops/.gitignore |    2
>  tools/testing/selftests/powerpc/copyloops/Makefile |    6 -
>  .../selftests/powerpc/copyloops/copy_mc_64.S       |    1
>  .../selftests/powerpc/copyloops/memcpy_mcsafe_64.S |    1
>  37 files changed, 452 insertions(+), 526 deletions(-)
>  rename arch/powerpc/lib/{memcpy_mcsafe_64.S => copy_mc_64.S} (98%)
>  create mode 100644 arch/x86/include/asm/copy_mc_test.h
>  delete mode 100644 arch/x86/include/asm/mcsafe_test.h
>  create mode 100644 arch/x86/lib/copy_mc.c
>  create mode 100644 arch/x86/lib/copy_mc_64.S
>  delete mode 100644 tools/arch/x86/include/asm/mcsafe_test.h
>  delete mode 100644 tools/perf/bench/mem-memcpy-x86-64-lib.c
>  create mode 120000 tools/testing/selftests/powerpc/copyloops/copy_mc_64.S
>  delete mode 120000 tools/testing/selftests/powerpc/copyloops/memcpy_mcsafe_64.S
>
> base-commit: ba4f184e126b751d1bffad5897f263108befc780
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