Re: [PATCH v2] prctl.2: Add PR_RISCV_SET_ICACHE_FLUSH_CTX

[Alejandro Colomar <alx@kernel.org>]

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Hi Charlie,

On Mon, Feb 12, 2024 at 09:19:21PM -0500, Charlie Jenkins wrote:
> I have proposed and documented the PR_RISCV_SET_ICACHE_FLUSH_CTX flag
> for prctl(2) to LKML. It has been reviewed and is expected to land
> during the Linux version 6.9 merge window. This adds the relevant
> documentation from that patch.
> 
> Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> ---
> I have proposed and documented the PR_RISCV_SET_ICACHE_FLUSH_CTX
> flag for prctl(2) to LKML. It has been reviewed and is expected to land
> during the Linux version 6.9 merge window.

Can you please ping when that happens?  The below LGTM.

Have a lovely night!
Alex

> ---
> Changes in v2:
> - Update formatting (Alejandro)
> - Link to v1: https://lore.kernel.org/r/20240124-fencei_prctl-v1-1-0bddafcef331@rivosinc.com
> ---
>  man2/prctl.2 | 59 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 59 insertions(+)
> 
> diff --git a/man2/prctl.2 b/man2/prctl.2
> index f1604a7cb..32ce8474f 100644
> --- a/man2/prctl.2
> +++ b/man2/prctl.2
> @@ -1147,6 +1147,65 @@ For further information, see the kernel source file
>  (or
>  .I Documentation/security/Yama.txt
>  before Linux 4.13).
> +.\" prctl PR_RISCV_SET_ICACHE_FLUSH_CTX
> +.TP
> +.BR PR_RISCV_SET_ICACHE_FLUSH_CTX " (since Linux 6.9, RISC-V only)"
> +Enable/disable icache flushing instructions in userspace.
> +The context and the scope can be provided using
> +.I arg2
> +and
> +.I arg3
> +respectively.
> +When scope is set to
> +.B PR_RISCV_SCOPE_PER_PROCESS
> +all threads in the process are permitted to emit icache flushing instructions.
> +Whenever any thread in the process is migrated, the corresponding hart's
> +icache will be guaranteed to be consistent with instruction storage.
> +This does not enforce any guarantees outside of migration.
> +If a thread modifies an instruction that another thread may attempt to
> +execute, the other thread must still emit an icache flushing instruction
> +before attempting to execute the potentially modified instruction.
> +This must be performed by the user-space program.
> +.IP
> +In per-thread context (eg. scope is set to
> +.BR PR_RISCV_SCOPE_PER_THREAD )
> +only the thread calling this function is permitted to emit icache flushing
> +instructions.
> +When the thread is migrated, the corresponding hart's icache will be
> +guaranteed to be consistent with instruction storage.
> +.IP
> +On kernels configured without SMP, this function is a nop as migrations across
> +harts will not occur.
> +.IP
> +The following values for
> +.I arg2
> +can be specified:
> +.RS
> +.TP
> +.BR PR_RISCV_CTX_SW_FENCEI_ON " (since Linux 6.9)"
> +Allow fence.i in user space.
> +.TP
> +.BR PR_RISCV_CTX_SW_FENCEI_OFF " (since Linux 6.9)"
> +Disallow fence.i in user space.
> +All threads in a process will be affected when scope is set to
> +.BR PR_RISCV_SCOPE_PER_PROCESS .
> +Therefore, caution must be taken; use this flag only when you can guarantee
> +that no thread in the process will emit fence.i from this point onward.
> +.RE
> +.IP
> +The following values for
> +.I arg3
> +can be specified:
> +.RS
> +.TP
> +.BR PR_RISCV_SCOPE_PER_PROCESS " (since Linux 6.9)"
> +Ensure the icache of any thread in this process is coherent with instruction
> +storage upon migration.
> +.TP
> +.BR PR_RISCV_SCOPE_PER_THREAD " (since Linux 6.9)"
> +Ensure the icache of the current thread is coherent with instruction storage
> +upon migration.
> +.RE
>  .\" prctl PR_SET_SECCOMP
>  .TP
>  .BR PR_SET_SECCOMP " (since Linux 2.6.23)"
> 
> ---
> base-commit: a81e893f2b9316869e6098c3a079c30a48158092
> change-id: 20240124-fencei_prctl-c24da2643379
> 
> Best regards,
> -- 
> Charlie Jenkins <charlie@rivosinc.com>
> 

-- 
<https://www.alejandro-colomar.es/>
Looking for a remote C programming job at the moment.

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