Re: [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and improve documentation

[Will Deacon]

On Tue, Jun 15, 2021 at 08:21:13PM -0700, Andy Lutomirski wrote:
> The old sync_core_before_usermode() comments suggested that a non-icache-syncing
> return-to-usermode instruction is x86-specific and that all other
> architectures automatically notice cross-modified code on return to
> userspace.
> 
> This is misleading.  The incantation needed to modify code from one
> CPU and execute it on another CPU is highly architecture dependent.
> On x86, according to the SDM, one must modify the code, issue SFENCE
> if the modification was WC or nontemporal, and then issue a "serializing
> instruction" on the CPU that will execute the code.  membarrier() can do
> the latter.
> 
> On arm64 and powerpc, one must flush the icache and then flush the pipeline
> on the target CPU, although the CPU manuals don't necessarily use this
> language.
> 
> So let's drop any pretense that we can have a generic way to define or
> implement membarrier's SYNC_CORE operation and instead require all
> architectures to define the helper and supply their own documentation as to
> how to use it.  This means x86, arm64, and powerpc for now.  Let's also
> rename the function from sync_core_before_usermode() to
> membarrier_sync_core_before_usermode() because the precise flushing details
> may very well be specific to membarrier, and even the concept of
> "sync_core" in the kernel is mostly an x86-ism.
> 
> (It may well be the case that, on real x86 processors, synchronizing the
>  icache (which requires no action at all) and "flushing the pipeline" is
>  sufficient, but trying to use this language would be confusing at best.
>  LFENCE does something awfully like "flushing the pipeline", but the SDM
>  does not permit LFENCE as an alternative to a "serializing instruction"
>  for this purpose.)
> 
> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> Cc: Paul Mackerras <paulus@xxxxxxxxx>
> Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
> Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 70216e18e519 ("membarrier: Provide core serializing command, *_SYNC_CORE")
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
>  .../membarrier-sync-core/arch-support.txt     | 68 ++++++-------------
>  arch/arm64/include/asm/sync_core.h            | 19 ++++++
>  arch/powerpc/include/asm/sync_core.h          | 14 ++++
>  arch/x86/Kconfig                              |  1 -
>  arch/x86/include/asm/sync_core.h              |  7 +-
>  arch/x86/kernel/alternative.c                 |  2 +-
>  arch/x86/kernel/cpu/mce/core.c                |  2 +-
>  arch/x86/mm/tlb.c                             |  3 +-
>  drivers/misc/sgi-gru/grufault.c               |  2 +-
>  drivers/misc/sgi-gru/gruhandles.c             |  2 +-
>  drivers/misc/sgi-gru/grukservices.c           |  2 +-
>  include/linux/sched/mm.h                      |  1 -
>  include/linux/sync_core.h                     | 21 ------
>  init/Kconfig                                  |  3 -
>  kernel/sched/membarrier.c                     | 15 ++--
>  15 files changed, 75 insertions(+), 87 deletions(-)
>  create mode 100644 arch/arm64/include/asm/sync_core.h
>  create mode 100644 arch/powerpc/include/asm/sync_core.h
>  delete mode 100644 include/linux/sync_core.h

For the arm64 bits (docs and asm/sync_core.h):

Acked-by: Will Deacon <will@xxxxxxxxxx>

Will