linux-kernel - Re: [PATCH v5 1/2] RISC-V: Cache SBI vendor values

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Date:   Mon, 10 Oct 2022 14:54:50 +0200
From:   Heiko Stuebner <heiko@...ech.de>
To:     Anup Patel <apatel@...tanamicro.com>
Cc:     atishp@...shpatra.org, anup@...infault.org, will@...nel.org,
        mark.rutland@....com, paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Conor.Dooley@...rochip.com,
        ajones@...tanamicro.com
Subject: Re: [PATCH v5 1/2] RISC-V: Cache SBI vendor values

Am Montag, 10. Oktober 2022, 14:45:45 CEST schrieb Anup Patel:
> On Mon, Oct 10, 2022 at 5:57 PM Heiko Stuebner <heiko@...ech.de> wrote:
> >
> > sbi_get_mvendorid(), sbi_get_marchid() and sbi_get_mimpid() might get
> > called multiple times, though the values of these CSRs should not change
> > during the runtime of a specific machine.
> >
> > So cache the values in the functions and prevent multiple ecalls
> > to read these values.
> >
> > As Andrew Jones noted, at least marchid and mimpid may be negative
> > values when viewed as a long, so we use a separate static bool to
> > indiciate the cached status.
> >
> > Suggested-by: Atish Patra <atishp@...shpatra.org>
> > Signed-off-by: Heiko Stuebner <heiko@...ech.de>
> > ---
> >  arch/riscv/kernel/sbi.c | 30 +++++++++++++++++++++++++++---
> >  1 file changed, 27 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/sbi.c b/arch/riscv/kernel/sbi.c
> > index 775d3322b422..cc618aaa9d11 100644
> > --- a/arch/riscv/kernel/sbi.c
> > +++ b/arch/riscv/kernel/sbi.c
> > @@ -625,17 +625,41 @@ static inline long sbi_get_firmware_version(void)
> >
> >  long sbi_get_mvendorid(void)
> >  {
> > -       return __sbi_base_ecall(SBI_EXT_BASE_GET_MVENDORID);
> > +       static long id;
> > +       static bool cached;
> > +
> > +       if (!cached) {
> > +               id = __sbi_base_ecall(SBI_EXT_BASE_GET_MVENDORID);
> > +               cached = true;
> > +       }
> > +
> > +       return id;
> >  }
> >
> >  long sbi_get_marchid(void)
> >  {
> > -       return __sbi_base_ecall(SBI_EXT_BASE_GET_MARCHID);
> > +       static long id;
> > +       static bool cached;
> 
> This breaks for heterogeneous SMP systems (similar to big.LITTLE)
> where HARTs will have different marchid even though they belong to
> same CPU Vendor.
> 
> Due to the above rationale, the patch adding marchid, mvendorid, and
> mimpid in /proc/cpuinfo caches these values on a per-CPU basis.

For people reading along, I think you mean
https://lore.kernel.org/r/20220727043829.151794-1-apatel@ventanamicro.com

For my understanding, was there a reason in the past for doing the caching
only for cpuinfo and not for every invocation of the ecalls?


Thanks
Heiko