Re: [RFC PATCH] membarrier: riscv: Provide core serializing command

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 8/3/23 11:45, Andrea Parri wrote:
> Adding Martin, Hans and Derek to the Cc: list,
> 
>    Andrea
> 
> 
> On Thu, Aug 03, 2023 at 06:01:11AM +0200, Andrea Parri wrote:
>> Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
>> Suggested-by: Palmer Dabbelt <palmer@dabbelt.com>
>> ---
>> For the MEMBARRIER maintainers:  RISC-V does not have "core serializing
>> instructions", meaning that there is no occurence of such a term in the
>> RISC-V ISA.  The discussion and git history about the SYNC_CORE command
>> suggested the implementation below: a FENCE.I instruction "synchronizes
>> the instruction and data streams" [1] on a CPU; in litmus parlance,
>>

Can you double-check that riscv switch_mm() implies a fence.i or 
equivalent on the CPU doing the switch_mm ?

AFAIR membarrier use of sync_core_before_usermode relies on switch_mm 
issuing a core serializing instruction.

Thanks,

Mathieu


>>    (single-hart test)
>>
>>    CPU0
>>
>>    UPDATE text   ;
>>    FENCE.I       ;
>>    EXECUTE text  ;  /* <-- will execute the updated/new text */
>>
>>
>>    (message-passing test)
>>
>>    CPU0             CPU1
>>
>>    UPDATE text   |  IF (flag) {     ;
>>    WMB           |    FENCE.I       ;
>>    SET flag      |    EXECUTE text  ;  /* execute the new text */
>>                  |  }               ;
>>
>>
>>    (and many others, including "maybe"s!  ;-) )
>>
>> How do these remarks resonate with the semantics of "a core serializing
>> instruction" (to be issued before returning to user-space)?
>>
>> RISCV maintainers, I'm missing some paths to user-space? (besides xRET)
>>
>>    Andrea
>>
>> [1] https://github.com/riscv/riscv-isa-manual/blob/main/src/zifencei.adoc
>>
>>
>>   .../sched/membarrier-sync-core/arch-support.txt   |  2 +-
>>   arch/riscv/Kconfig                                |  2 ++
>>   arch/riscv/include/asm/sync_core.h                | 15 +++++++++++++++
>>   3 files changed, 18 insertions(+), 1 deletion(-)
>>   create mode 100644 arch/riscv/include/asm/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 23260ca449468..a17117d76e6d8 100644
>> --- a/Documentation/features/sched/membarrier-sync-core/arch-support.txt
>> +++ b/Documentation/features/sched/membarrier-sync-core/arch-support.txt
>> @@ -44,7 +44,7 @@
>>       |    openrisc: | TODO |
>>       |      parisc: | TODO |
>>       |     powerpc: |  ok  |
>> -    |       riscv: | TODO |
>> +    |       riscv: |  ok  |
>>       |        s390: |  ok  |
>>       |          sh: | TODO |
>>       |       sparc: | TODO |
>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>> index 4c07b9189c867..ed7ddaedc692e 100644
>> --- a/arch/riscv/Kconfig
>> +++ b/arch/riscv/Kconfig
>> @@ -27,6 +27,7 @@ config RISCV
>>   	select ARCH_HAS_GCOV_PROFILE_ALL
>>   	select ARCH_HAS_GIGANTIC_PAGE
>>   	select ARCH_HAS_KCOV
>> +	select ARCH_HAS_MEMBARRIER_SYNC_CORE
>>   	select ARCH_HAS_MMIOWB
>>   	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
>>   	select ARCH_HAS_PMEM_API
>> @@ -35,6 +36,7 @@ config RISCV
>>   	select ARCH_HAS_SET_MEMORY if MMU
>>   	select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
>>   	select ARCH_HAS_STRICT_MODULE_RWX if MMU && !XIP_KERNEL
>> +	select ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
>>   	select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
>>   	select ARCH_HAS_UBSAN_SANITIZE_ALL
>>   	select ARCH_HAS_VDSO_DATA
>> diff --git a/arch/riscv/include/asm/sync_core.h b/arch/riscv/include/asm/sync_core.h
>> new file mode 100644
>> index 0000000000000..d3ec6ac47ac9b
>> --- /dev/null
>> +++ b/arch/riscv/include/asm/sync_core.h
>> @@ -0,0 +1,15 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +#ifndef _ASM_RISCV_SYNC_CORE_H
>> +#define _ASM_RISCV_SYNC_CORE_H
>> +
>> +/*
>> + * Ensure that a core serializing instruction is issued before returning
>> + * to user-mode.  RISC-V implements return to user-space through an xRET
>> + * instruction, which is not core serializing.
>> + */
>> +static inline void sync_core_before_usermode(void)
>> +{
>> +	asm volatile ("fence.i" ::: "memory");
>> +}
>> +
>> +#endif /* _ASM_RISCV_SYNC_CORE_H */
>> -- 
>> 2.34.1
>>

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com