Re: [PATCH V11 09/17] riscv: qspinlock: errata: Add ERRATA_THEAD_WRITE_ONCE fixup

[Leonardo Bras <leobras@redhat.com>]

On Sun, Sep 17, 2023 at 11:15:51PM +0800, Guo Ren wrote:
> On Thu, Sep 14, 2023 at 4:32 PM Leonardo Bras <leobras@redhat.com> wrote:
> >
> > On Sun, Sep 10, 2023 at 04:29:03AM -0400, guoren@kernel.org wrote:
> > > From: Guo Ren <guoren@linux.alibaba.com>
> > >
> > > The early version of T-Head C9xx cores has a store merge buffer
> > > delay problem. The store merge buffer could improve the store queue
> > > performance by merging multi-store requests, but when there are not
> > > continued store requests, the prior single store request would be
> > > waiting in the store queue for a long time. That would cause
> > > significant problems for communication between multi-cores. This
> > > problem was found on sg2042 & th1520 platforms with the qspinlock
> > > lock torture test.
> > >
> > > So appending a fence w.o could immediately flush the store merge
> > > buffer and let other cores see the write result.
> > >
> > > This will apply the WRITE_ONCE errata to handle the non-standard
> > > behavior via appending a fence w.o instruction for WRITE_ONCE().
> > >
> > > Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> > > Signed-off-by: Guo Ren <guoren@kernel.org>
> > > ---
> > >  arch/riscv/Kconfig.errata              | 19 +++++++++++++++++++
> > >  arch/riscv/errata/thead/errata.c       | 20 ++++++++++++++++++++
> > >  arch/riscv/include/asm/errata_list.h   | 13 -------------
> > >  arch/riscv/include/asm/rwonce.h        | 24 ++++++++++++++++++++++++
> > >  arch/riscv/include/asm/vendorid_list.h | 14 ++++++++++++++
> > >  include/asm-generic/rwonce.h           |  2 ++
> > >  6 files changed, 79 insertions(+), 13 deletions(-)
> > >  create mode 100644 arch/riscv/include/asm/rwonce.h
> > >
> > > diff --git a/arch/riscv/Kconfig.errata b/arch/riscv/Kconfig.errata
> > > index 1aa85a427ff3..c919cc3f1a3a 100644
> > > --- a/arch/riscv/Kconfig.errata
> > > +++ b/arch/riscv/Kconfig.errata
> > > @@ -77,4 +77,23 @@ config ERRATA_THEAD_PMU
> > >
> > >         If you don't know what to do here, say "Y".
> > >
> > > +config ERRATA_THEAD_WRITE_ONCE
> > > +     bool "Apply T-Head WRITE_ONCE errata"
> > > +     depends on ERRATA_THEAD
> > > +     default y
> > > +     help
> > > +       The early version of T-Head C9xx cores has a store merge buffer
> > > +       delay problem. The store merge buffer could improve the store queue
> > > +       performance by merging multi-store requests, but when there are no
> > > +       continued store requests, the prior single store request would be
> > > +       waiting in the store queue for a long time. That would cause
> > > +       significant problems for communication between multi-cores. Appending
> > > +       a fence w.o could immediately flush the store merge buffer and let
> > > +       other cores see the write result.
> > > +
> > > +       This will apply the WRITE_ONCE errata to handle the non-standard
> > > +       behavior via appending a fence w.o instruction for WRITE_ONCE().
> > > +
> > > +       If you don't know what to do here, say "Y".
> > > +
> > >  endmenu # "CPU errata selection"
> > > diff --git a/arch/riscv/errata/thead/errata.c b/arch/riscv/errata/thead/errata.c
> > > index be84b14f0118..751eb5a7f614 100644
> > > --- a/arch/riscv/errata/thead/errata.c
> > > +++ b/arch/riscv/errata/thead/errata.c
> > > @@ -69,6 +69,23 @@ static bool errata_probe_pmu(unsigned int stage,
> > >       return true;
> > >  }
> > >
> > > +static bool errata_probe_write_once(unsigned int stage,
> > > +                                 unsigned long arch_id, unsigned long impid)
> > > +{
> > > +     if (!IS_ENABLED(CONFIG_ERRATA_THEAD_WRITE_ONCE))
> > > +             return false;
> > > +
> > > +     /* target-c9xx cores report arch_id and impid as 0 */
> > > +     if (arch_id != 0 || impid != 0)
> > > +             return false;
> > > +
> > > +     if (stage == RISCV_ALTERNATIVES_BOOT ||
> > > +         stage == RISCV_ALTERNATIVES_MODULE)
> > > +             return true;
> > > +
> > > +     return false;
> > > +}
> > > +
> > >  static u32 thead_errata_probe(unsigned int stage,
> > >                             unsigned long archid, unsigned long impid)
> > >  {
> > > @@ -83,6 +100,9 @@ static u32 thead_errata_probe(unsigned int stage,
> > >       if (errata_probe_pmu(stage, archid, impid))
> > >               cpu_req_errata |= BIT(ERRATA_THEAD_PMU);
> > >
> > > +     if (errata_probe_write_once(stage, archid, impid))
> > > +             cpu_req_errata |= BIT(ERRATA_THEAD_WRITE_ONCE);
> > > +
> > >       return cpu_req_errata;
> > >  }
> > >
> > > diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
> > > index 712cab7adffe..fbb2b8d39321 100644
> > > --- a/arch/riscv/include/asm/errata_list.h
> > > +++ b/arch/riscv/include/asm/errata_list.h
> > > @@ -11,19 +11,6 @@
> > >  #include <asm/hwcap.h>
> > >  #include <asm/vendorid_list.h>
> > >
> > > -#ifdef CONFIG_ERRATA_SIFIVE
> > > -#define      ERRATA_SIFIVE_CIP_453 0
> > > -#define      ERRATA_SIFIVE_CIP_1200 1
> > > -#define      ERRATA_SIFIVE_NUMBER 2
> > > -#endif
> > > -
> > > -#ifdef CONFIG_ERRATA_THEAD
> > > -#define      ERRATA_THEAD_PBMT 0
> > > -#define      ERRATA_THEAD_CMO 1
> > > -#define      ERRATA_THEAD_PMU 2
> > > -#define      ERRATA_THEAD_NUMBER 3
> > > -#endif
> > > -
> >
> > Here I understand you are moving stuff from errata_list.h to
> > vendorid_list.h. Wouldn't it be better to do this on a separated patch
> > before this one?
> Okay.
> 
> >
> > I understand this is used here, but it looks like it's unrelated.
> >
> > >  #ifdef __ASSEMBLY__
> > >
> > >  #define ALT_INSN_FAULT(x)                                            \
> > > diff --git a/arch/riscv/include/asm/rwonce.h b/arch/riscv/include/asm/rwonce.h
> > > new file mode 100644
> > > index 000000000000..be0b8864969d
> > > --- /dev/null
> > > +++ b/arch/riscv/include/asm/rwonce.h
> > > @@ -0,0 +1,24 @@
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > +
> > > +#ifndef __ASM_RWONCE_H
> > > +#define __ASM_RWONCE_H
> > > +
> > > +#include <linux/compiler_types.h>
> > > +#include <asm/alternative-macros.h>
> > > +#include <asm/vendorid_list.h>
> > > +
> > > +#define __WRITE_ONCE(x, val)                         \
> > > +do {                                                 \
> > > +     *(volatile typeof(x) *)&(x) = (val);            \
> > > +     asm volatile(ALTERNATIVE(                       \
> > > +             __nops(1),                              \
> > > +             "fence w, o\n\t",                       \
> > > +             THEAD_VENDOR_ID,                        \
> > > +             ERRATA_THEAD_WRITE_ONCE,                \
> > > +             CONFIG_ERRATA_THEAD_WRITE_ONCE)         \
> > > +             : : : "memory");                        \
> > > +} while (0)
> > > +
> > > +#include <asm-generic/rwonce.h>
> > > +
> > > +#endif       /* __ASM_RWONCE_H */
> >
> > IIUC the idea here is to have an alternative __WRITE_ONCE that replaces the
> > asm-generic one.
> >
> > Honestly, this asm alternative here seems too much information, and too
> > cryptic. I mean, yeah in the patch it all makes sense, but I imagine myself
> > in the future looking at all this and trying to understand what is going
> > on.
> >
> > Wouldn't it look better to have something like:
> >
> > #####
> >
> > /* Some explanation like the one on Kconfig */
> >
> > #define write_once_flush()                      \
> > do {                                            \
> >         asm volatile(ALTERNATIVE(                       \
> >                 __nops(1),                      \
> >                 "fence w, o\n\t",               \
> >                 THEAD_VENDOR_ID,                \
> >                 ERRATA_THEAD_WRITE_ONCE,        \
> >                 CONFIG_ERRATA_THEAD_WRITE_ONCE) \
> >                 : : : "memory");                \
> > } while(0)
> >
> >
> > #define __WRITE_ONCE(x, val)                    \
> > do {                                            \
> >         *(volatile typeof(x) *)&(x) = (val);    \
> >         write_once_flush();                     \
> > } while(0)
> >
> > #####
> >
> >
> > This way I could quickly see there is a flush after the writting of
> > WRITE_ONCE(), and this flush is the above "complicated" asm.
> >
> > What do you think?
> Okay, good point, and I would take it.

Thanks!

Once you take the above suggestions, please include in your next patch:

Reviewed-by: Leonardo Bras <leobras@redhat.com>


> 
> >
> > > diff --git a/arch/riscv/include/asm/vendorid_list.h b/arch/riscv/include/asm/vendorid_list.h
> > > index cb89af3f0704..73078cfe4029 100644
> > > --- a/arch/riscv/include/asm/vendorid_list.h
> > > +++ b/arch/riscv/include/asm/vendorid_list.h
> > > @@ -8,4 +8,18 @@
> > >  #define SIFIVE_VENDOR_ID     0x489
> > >  #define THEAD_VENDOR_ID              0x5b7
> > >
> > > +#ifdef CONFIG_ERRATA_SIFIVE
> > > +#define      ERRATA_SIFIVE_CIP_453 0
> > > +#define      ERRATA_SIFIVE_CIP_1200 1
> > > +#define      ERRATA_SIFIVE_NUMBER 2
> > > +#endif
> > > +
> > > +#ifdef CONFIG_ERRATA_THEAD
> > > +#define      ERRATA_THEAD_PBMT 0
> > > +#define      ERRATA_THEAD_CMO 1
> > > +#define      ERRATA_THEAD_PMU 2
> > > +#define      ERRATA_THEAD_WRITE_ONCE 3
> > > +#define      ERRATA_THEAD_NUMBER 4
> > > +#endif
> > > +
> > >  #endif
> > > diff --git a/include/asm-generic/rwonce.h b/include/asm-generic/rwonce.h
> > > index 8d0a6280e982..fb07fe8c6e45 100644
> > > --- a/include/asm-generic/rwonce.h
> > > +++ b/include/asm-generic/rwonce.h
> > > @@ -50,10 +50,12 @@
> > >       __READ_ONCE(x);                                                 \
> > >  })
> > >
> > > +#ifndef __WRITE_ONCE
> > >  #define __WRITE_ONCE(x, val)                                         \
> > >  do {                                                                 \
> > >       *(volatile typeof(x) *)&(x) = (val);                            \
> > >  } while (0)
> > > +#endif
> > >
> > >  #define WRITE_ONCE(x, val)                                           \
> > >  do {                                                                 \
> > > --
> > > 2.36.1
> > >
> >
> 
> 
> -- 
> Best Regards
>  Guo Ren
>