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

Nicholas Piggin
2021-06-16T04:47:48Z

Excerpts from Andy Lutomirski's message of June 16, 2021 1:21 pm:
> 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
>
> diff --git a/Documentation/features/sched/membarrier-sync-core/arch-support.txt b/Documentation/features/sched/membarrier-sync-core/arch-support.txt
> index 883d33b265d6..41c9ebcb275f 100644
> --- a/Documentation/features/sched/membarrier-sync-core/arch-support.txt
> +++ b/Documentation/features/sched/membarrier-sync-core/arch-support.txt
> @@ -5,51 +5,25 @@
> #
> # Architecture requirements
> #
> -# * arm/arm64/powerpc
> #
> -# Rely on implicit context synchronization as a result of exception return
> -# when returning from IPI handler, and when returning to user-space.
> -#
> -# * x86
> -#
> -# x86-32 uses IRET as return from interrupt, which takes care of the IPI.
> -# However, it uses both IRET and SYSEXIT to go back to user-space. The IRET
> -# instruction is core serializing, but not SYSEXIT.
> -#
> -# x86-64 uses IRET as return from interrupt, which takes care of the IPI.
> -# However, it can return to user-space through either SYSRETL (compat code),
> -# SYSRETQ, or IRET.
> -#
> -# Given that neither SYSRET{L,Q}, nor SYSEXIT, are core serializing, we rely
> -# instead on write_cr3() performed by switch_mm() to provide core serialization
> -# after changing the current mm, and deal with the special case of kthread ->
> -# uthread (temporarily keeping current mm into active_mm) by issuing a
> -# sync_core_before_usermode() in that specific case.
> -#
> - -----------------------
> - | arch |status|
> - -----------------------
> - | alpha: | TODO |
> - | arc: | TODO |
> - | arm: | ok |
> - | arm64: | ok |
> - | csky: | TODO |
> - | h8300: | TODO |
> - | hexagon: | TODO |
> - | ia64: | TODO |
> - | m68k: | TODO |
> - | microblaze: | TODO |
> - | mips: | TODO |
> - | nds32: | TODO |
> - | nios2: | TODO |
> - | openrisc: | TODO |
> - | parisc: | TODO |
> - | powerpc: | ok |
> - | riscv: | TODO |
> - | s390: | TODO |
> - | sh: | TODO |
> - | sparc: | TODO |
> - | um: | TODO |
> - | x86: | ok |
> - | xtensa: | TODO |
> - -----------------------
> +# An architecture that wants to support
> +# MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE needs to define precisely what it
> +# is supposed to do and implement membarrier_sync_core_before_usermode() to
> +# make it do that. Then it can select ARCH_HAS_MEMBARRIER_SYNC_CORE via
> +# Kconfig.Unfortunately, MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE is not a
> +# fantastic API and may not make sense on all architectures. Once an
> +# architecture meets these requirements,
> +#
> +# On x86, a program can safely modify code, issue
> +# MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE, and then execute that code, via
> +# the modified address or an alias, from any thread in the calling process.
> +#
> +# On arm64, a program can modify code, flush the icache as needed, and issue
> +# MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE to force a "context synchronizing
> +# event", aka pipeline flush on all CPUs that might run the calling process.
> +# Then the program can execute the modified code as long as it is executed
> +# from an address consistent with the icache flush and the CPU's cache type.
> +#
> +# On powerpc, a program can use MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE
> +# similarly to arm64. It would be nice if the powerpc maintainers could
> +# add a more clear explanantion.
> diff --git a/arch/arm64/include/asm/sync_core.h b/arch/arm64/include/asm/sync_core.h
> new file mode 100644
> index 000000000000..74996bf533bb
> --- /dev/null
> +++ b/arch/arm64/include/asm/sync_core.h
> @@ -0,0 +1,19 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_ARM64_SYNC_CORE_H
> +#define _ASM_ARM64_SYNC_CORE_H
> +
> +#include <asm/barrier.h>
> +
> +/*
> + * On arm64, anyone trying to use membarrier() to handle JIT code is
> + * required to first flush the icache and then do SYNC_CORE. All that's
> + * needed after the icache flush is to execute a "context synchronization
> + * event". Right now, ERET does this, and we are guaranteed to ERET before
> + * any user code runs. If Linux ever programs the CPU to make ERET stop
> + * being a context synchronizing event, then this will need to be adjusted.
> + */
> +static inline void membarrier_sync_core_before_usermode(void)
> +{
> +}
> +
> +#endif /* _ASM_ARM64_SYNC_CORE_H */
> diff --git a/arch/powerpc/include/asm/sync_core.h b/arch/powerpc/include/asm/sync_core.h
> new file mode 100644
> index 000000000000..589fdb34beab
> --- /dev/null
> +++ b/arch/powerpc/include/asm/sync_core.h
> @@ -0,0 +1,14 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_POWERPC_SYNC_CORE_H
> +#define _ASM_POWERPC_SYNC_CORE_H
> +
> +#include <asm/barrier.h>
> +
> +/*
> + * XXX: can a powerpc person put an appropriate comment here?
> + */
> +static inline void membarrier_sync_core_before_usermode(void)
> +{
> +}
> +
> +#endif /* _ASM_POWERPC_SYNC_CORE_H */

powerpc's can just go in asm/membarrier.h

/*
* The RFI family of instructions are context synchronising, and
* that is how we return to userspace, so nothing is required here.
*/

> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index c32c32a2441e..f72a6ab3fac2 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -5,6 +5,9 @@
> * membarrier system call
> */
> #include "sched.h"
> +#ifdef CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE
> +#include <asm/sync_core.h>
> +#endif

Can you

#else
static inline void membarrier_sync_core_before_usermode(void)
{
/* this gets constant folded out */
}
#endif

And avoid adding the ifdefs in the following code?

Otherwise I think this is good.

Acked-by: Nicholas Piggin <npiggin@gmail.com>

Thanks,
Nick

>
> /*
> * The basic principle behind the regular memory barrier mode of membarrier()
> @@ -221,6 +224,7 @@ static void ipi_mb(void *info)
> smp_mb(); /* IPIs should be serializing but paranoid. */
> }
>
> +#ifdef CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE
> static void ipi_sync_core(void *info)
> {
> /*
> @@ -230,13 +234,14 @@ static void ipi_sync_core(void *info)
> * the big comment at the top of this file.
> *
> * A sync_core() would provide this guarantee, but
> - * sync_core_before_usermode() might end up being deferred until
> - * after membarrier()'s smp_mb().
> + * membarrier_sync_core_before_usermode() might end up being deferred
> + * until after membarrier()'s smp_mb().
> */
> smp_mb(); /* IPIs should be serializing but paranoid. */
>
> - sync_core_before_usermode();
> + membarrier_sync_core_before_usermode();
> }
> +#endif
>
> static void ipi_rseq(void *info)
> {
> @@ -368,12 +373,14 @@ static int membarrier_private_expedited(int flags, int cpu_id)
> smp_call_func_t ipi_func = ipi_mb;
>
> if (flags == MEMBARRIER_FLAG_SYNC_CORE) {
> - if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
> +#ifndef CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE
> return -EINVAL;
> +#else
> if (!(atomic_read(&mm->membarrier_state) &
> MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY))
> return -EPERM;
> ipi_func = ipi_sync_core;
> +#endif
> } else if (flags == MEMBARRIER_FLAG_RSEQ) {
> if (!IS_ENABLED(CONFIG_RSEQ))
> return -EINVAL;
> --
> 2.31.1