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From:   Heiko Stuebner <heiko@sntech.de>
To:     Anup Patel <apatel@ventanamicro.com>
Cc:     atishp@atishpatra.org, anup@brainfault.org, will@kernel.org,
        mark.rutland@arm.com, paul.walmsley@sifive.com, palmer@dabbelt.com,
        aou@eecs.berkeley.edu, linux-riscv@lists.infradead.org,
        linux-kernel@vger.kernel.org, Conor.Dooley@microchip.com,
        ajones@ventanamicro.com
Subject: Re: [PATCH v5 1/2] RISC-V: Cache SBI vendor values
Date:   Tue, 11 Oct 2022 09:58:38 +0200
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Hi Anup,

Am Montag, 10. Oktober 2022, 15:14:21 CEST schrieb Anup Patel:
> On Mon, Oct 10, 2022 at 6:25 PM Heiko Stuebner <heiko@sntech.de> wrote:
> >
> > Am Montag, 10. Oktober 2022, 14:45:45 CEST schrieb Anup Patel:
> > > On Mon, Oct 10, 2022 at 5:57 PM Heiko Stuebner <heiko@sntech.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@atishpatra.org>
> > > > Signed-off-by: Heiko Stuebner <heiko@sntech.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
> 
> Yes, this is the patch I am referring.
> 
> >
> > 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?
> 
> The caching was done only for /proc/cpuinfo because at that point
> time only "cat /proc/cpuinfo" would need these value at runtime.
> 
> Now that we have more uses of marchid, mvendorid, and mimpid,
> at runtime, we should definitely have the cached values available
> to other parts of kernel.
> 
> My suggestion is to keep sbi_get_xyz() functions unmodified and
> instead add new functions in arch/riscv/kernel/cpu.c (on-top-of
> /proc/cpuinfo patch) which allows users to read cached values of
> any CPU.
> 
> For example, we could export following functions from
> arch/riscv/kernel/cpu.c:
> unsigned long riscv_cached_mvendorid(unsigned int cpu);
> unsigned long riscv_cached_marchid(unsigned int cpu);
> unsigned long riscv_cached_mimpid(unsigned int cpu);

sounds sensible, so I'll go that way you suggested :-)


Thanks
Heiko