[RFC PATCH] riscv: Add Zawrs support for spinlocks

[Christoph Müllner]

On Thu, Jun 2, 2022 at 6:24 PM Randy Dunlap <rdunlap at infradead.org> wrote:
>
> Hi--
>
> On 6/2/22 07:10, Christoph Muellner wrote:
> > The current RISC-V code uses the generic ticket lock implementation,
> > that calls the macros smp_cond_load_relaxed() and smp_cond_load_acquire().
> > Currently, RISC-V uses the generic implementation of these macros.
> > This patch introduces a RISC-V specific implementation, of these
> > macros, that peels off the first loop iteration and modifies the waiting
> > loop such, that it is possible to use the WRS instruction of the Zawrs
> > ISA extension to stall the CPU.
> >
> > The resulting implementation of smp_cond_load_*() will only work for
> > 32-bit or 64-bit types for RV64 and 32-bit types for RV32.
> > This is caused by the restrictions of the LR instruction (RISC-V only
> > has LR.W and LR.D). Compiler assertions guard this new restriction.
> >
> > This patch uses the existing RISC-V ISA extension framework
> > to detect the presents of Zawrs at run-time.
> > If available a NOP instruction will be replaced by WRS.
> > A similar patch could add support for the PAUSE instruction of
> > the Zihintpause ISA extension.
> >
> > The whole mechanism is gated by Kconfig setting, which defaults to Y.
> >
> > The Zawrs specification can be found here:
> > https://github.com/riscv/riscv-zawrs/blob/main/zawrs.adoc
> >
> > Note, that the Zawrs extension is not frozen or ratified yet.
> > Therefore this patch is an RFC and not intended to get merged.
> >
> > Signed-off-by: Christoph Muellner <christoph.muellner at vrull.io>
> > ---
> >  arch/riscv/Kconfig                   | 10 +++
> >  arch/riscv/include/asm/barrier.h     | 97 ++++++++++++++++++++++++++++
> >  arch/riscv/include/asm/errata_list.h | 12 +++-
> >  arch/riscv/include/asm/hwcap.h       |  3 +-
> >  arch/riscv/kernel/cpu.c              |  1 +
> >  arch/riscv/kernel/cpufeature.c       | 13 ++++
> >  6 files changed, 133 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 905e550e0fd3..054872317d4a 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -358,6 +358,16 @@ config RISCV_ISA_C
> >
> >          If you don't know what to do here, say Y.
> >
> > +config RISCV_ISA_ZAWRS
> > +     bool "Zawrs extension support"
> > +     select RISCV_ALTERNATIVE
> > +     default y
> > +     help
> > +        Adds support to dynamically detect the presence of the Zawrs extension
> > +        (wait for reservation set) and enable its usage.
> > +
> > +        If you don't know what to do here, say Y.
> > +
>
> With this patch, it is possible to enable XIP_KERNEL and
> RISCV_ISA_ZAWRS at the same time.  That causes a kconfig warning:
>
> WARNING: unmet direct dependencies detected for RISCV_ALTERNATIVE
>   Depends on [n]: !XIP_KERNEL [=y]
>   Selected by [y]:
>   - RISCV_ISA_ZAWRS [=y]
>   - RISCV_ISA_SVPBMT [=y] && 64BIT [=y] && MMU [=y]
>
> because RISCV_ALTERNATIVE depends on !XIP_KERNEL:
>
> config RISCV_ALTERNATIVE
>         bool
>         depends on !XIP_KERNEL

I will add a "depends on !XIP_KERNEL" to RISCV_ISA_ZAWRS.

Thanks!

>
>
> >  config RISCV_ISA_SVPBMT
> >       bool "SVPBMT extension support"
> >       depends on 64BIT && MMU
>
>
> --
> ~Randy