[RFC 1/2] riscv: process: Introduce idle thread using Zawrs extension

[Andrew Jones]

On Thu, Apr 18, 2024 at 04:05:55PM +0100, Conor Dooley wrote:
> + Drew,
> 
> On Thu, Apr 18, 2024 at 07:49:41PM +0800, Xu Lu wrote:
> > The Zawrs extension introduces a new instruction WRS.NTO, which will
> > register a reservation set and causes the hart to temporarily stall
> > execution in a low-power state until a store occurs to the reservation
> > set or an interrupt is observed.
> > 
> > This commit implements new version of idle thread for RISC-V via Zawrs
> > extension.
> > 
> > Signed-off-by: Xu Lu <luxu.kernel at bytedance.com>
> > Reviewed-by: Hangjing Li <lihangjing at bytedance.com>
> > Reviewed-by: Liang Deng <dengliang.1214 at bytedance.com>
> > Reviewed-by: Wen Chai <chaiwen.cc at bytedance.com>
> > ---
> >  arch/riscv/Kconfig                 | 24 +++++++++++++++++
> >  arch/riscv/include/asm/cpuidle.h   | 11 +-------
> >  arch/riscv/include/asm/hwcap.h     |  1 +
> >  arch/riscv/include/asm/processor.h | 17 +++++++++++++
> >  arch/riscv/kernel/cpu.c            |  5 ++++
> >  arch/riscv/kernel/cpufeature.c     |  1 +
> >  arch/riscv/kernel/process.c        | 41 +++++++++++++++++++++++++++++-
> >  7 files changed, 89 insertions(+), 11 deletions(-)
> > 
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index be09c8836d56..a0d344e9803f 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -19,6 +19,7 @@ config RISCV
> >  	select ARCH_ENABLE_SPLIT_PMD_PTLOCK if PGTABLE_LEVELS > 2
> >  	select ARCH_ENABLE_THP_MIGRATION if TRANSPARENT_HUGEPAGE
> >  	select ARCH_HAS_BINFMT_FLAT
> > +	select ARCH_HAS_CPU_FINALIZE_INIT
> >  	select ARCH_HAS_CURRENT_STACK_POINTER
> >  	select ARCH_HAS_DEBUG_VIRTUAL if MMU
> >  	select ARCH_HAS_DEBUG_VM_PGTABLE
> > @@ -525,6 +526,20 @@ config RISCV_ISA_SVPBMT
> >  
> >  	   If you don't know what to do here, say Y.
> >  
> > +config RISCV_ISA_ZAWRS
> > +	bool "Zawrs extension support for wait-on-reservation-set instructions"
> > +	depends on RISCV_ALTERNATIVE
> > +	default y
> > +	help
> > +	   Adds support to dynamically detect the presence of the Zawrs
> > +	   extension and enable its usage.
> 
> Drew, could you, in your update, use the wording:
> 	   Add support for enabling optimisations in the kernel when the
> 	   Zawrs extension is detected at boot.

How about

  The Zawrs extension defines a pair of instructions to be used in
  polling loops which allow a hart to enter a low-power state or to
  trap to the hypervisor while waiting on a store to a memory location.
  Enable the use of these instructions when the Zawrs extension is
  detected at boot.

Thanks,
drew