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

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@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <redacted>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: x86@kernel.org
Cc: stable@vger.kernel.org
Fixes: 70216e18e519 ("membarrier: Provide core serializing command, *_SYNC_CORE")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 .../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@kernel.org>

Will

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