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

[Charlie Jenkins <charlie@rivosinc.com>]

On Sun, Feb 18, 2024 at 02:15:53AM +0100, Alejandro Colomar wrote:
> 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.

Sure thing. Thank you!

> 
> 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.