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

Will Deacon
2021-06-16T10:22:16Z

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 <paulus@samba.org>
> 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