Re: [PATCH V11 13/17] RISC-V: paravirt: pvqspinlock: Add SBI implementation

[Leonardo Bras <leobras@redhat.com>]

On Sun, Sep 17, 2023 at 11:06:48PM +0800, Guo Ren wrote:
> On Fri, Sep 15, 2023 at 2:23 PM Leonardo Bras <leobras@redhat.com> wrote:
> >
> > On Sun, Sep 10, 2023 at 04:29:07AM -0400, guoren@kernel.org wrote:
> > > From: Guo Ren <guoren@linux.alibaba.com>
> > >
> > > Implement pv_kick with SBI implementation, and add SBI_EXT_PVLOCK
> > > extension detection.
> > >
> > > Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> > > Signed-off-by: Guo Ren <guoren@kernel.org>
> > > ---
> > >  arch/riscv/include/asm/sbi.h           | 6 ++++++
> > >  arch/riscv/kernel/qspinlock_paravirt.c | 7 ++++++-
> > >  2 files changed, 12 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/riscv/include/asm/sbi.h b/arch/riscv/include/asm/sbi.h
> > > index e0233b3d7a5f..3533f8d4f3e2 100644
> > > --- a/arch/riscv/include/asm/sbi.h
> > > +++ b/arch/riscv/include/asm/sbi.h
> > > @@ -30,6 +30,7 @@ enum sbi_ext_id {
> > >       SBI_EXT_HSM = 0x48534D,
> > >       SBI_EXT_SRST = 0x53525354,
> > >       SBI_EXT_PMU = 0x504D55,
> > > +     SBI_EXT_PVLOCK = 0xAB0401,
> > >
> > >       /* Experimentals extensions must lie within this range */
> > >       SBI_EXT_EXPERIMENTAL_START = 0x08000000,
> > > @@ -243,6 +244,11 @@ enum sbi_pmu_ctr_type {
> > >  /* Flags defined for counter stop function */
> > >  #define SBI_PMU_STOP_FLAG_RESET (1 << 0)
> > >
> > > +/* SBI PVLOCK (kick cpu out of wfi) */
> > > +enum sbi_ext_pvlock_fid {
> > > +     SBI_EXT_PVLOCK_KICK_CPU = 0,
> > > +};
> > > +
> > >  #define SBI_SPEC_VERSION_DEFAULT     0x1
> > >  #define SBI_SPEC_VERSION_MAJOR_SHIFT 24
> > >  #define SBI_SPEC_VERSION_MAJOR_MASK  0x7f
> > > diff --git a/arch/riscv/kernel/qspinlock_paravirt.c b/arch/riscv/kernel/qspinlock_paravirt.c
> > > index a0ad4657f437..571626f350be 100644
> > > --- a/arch/riscv/kernel/qspinlock_paravirt.c
> > > +++ b/arch/riscv/kernel/qspinlock_paravirt.c
> > > @@ -11,6 +11,8 @@
> > >
> > >  void pv_kick(int cpu)
> > >  {
> > > +     sbi_ecall(SBI_EXT_PVLOCK, SBI_EXT_PVLOCK_KICK_CPU,
> > > +               cpuid_to_hartid_map(cpu), 0, 0, 0, 0, 0);
> > >       return;
> > >  }
> > >
> > > @@ -25,7 +27,7 @@ void pv_wait(u8 *ptr, u8 val)
> > >       if (READ_ONCE(*ptr) != val)
> > >               goto out;
> > >
> > > -     /* wait_for_interrupt(); */
> > > +     wait_for_interrupt();
> > >  out:
> > >       local_irq_restore(flags);
> > >  }
> > > @@ -62,6 +64,9 @@ void __init pv_qspinlock_init(void)
> > >       if(sbi_get_firmware_id() != SBI_EXT_BASE_IMPL_ID_KVM)
> > >               return;
> > >
> > > +     if (!sbi_probe_extension(SBI_EXT_PVLOCK))
> > > +             return;
> > > +
> > >       pr_info("PV qspinlocks enabled\n");
> > >       __pv_init_lock_hash();
> > >
> > > --
> > > 2.36.1
> > >
> >
> > IIUC this PVLOCK extension is now a requirement to use pv_qspinlock(), and
> > it allows a cpu to use an instruction to wait for interrupt in pv_wait(),
> > and kicks it out of this wait using a new sbi_ecall() on pv_kick().
> Yes.
> 
> >
> > Overall it LGTM, but would be nice to have the reference doc in the commit
> > msg. I end up inferring some of the inner workings by your implementation,
> > which is not ideal for reviewing.
> I would improve the commit msg in the next version of patch.

Thx!
Leo

> 
> >
> > If understanding above is right,
> > Reviewed-by: Leonardo Bras <leobras@redhat.com>
> >
> > Thanks!
> > Leo
> >
> 
> 
> -- 
> Best Regards
>  Guo Ren
>