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

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Date:   Thu, 06 Oct 2022 01:07:00 +0200
From:   Heiko Stuebner <heiko@...ech.de>
To:     Andrew Jones <ajones@...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,
        cmuellner@...ux.com, samuel@...lland.org
Subject: Re: [PATCH 1/2] RISC-V: Cache SBI vendor values

Hi Drew,

Am Mittwoch, 5. Oktober 2022, 19:07:02 CEST schrieb Andrew Jones:
> On Tue, Oct 04, 2022 at 10:37:23PM +0200, Heiko Stuebner 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.
> > 
> > Suggested-by: Atish Patra <atishp@...shpatra.org>
> > Signed-off-by: Heiko Stuebner <heiko@...ech.de>
> > ---
> >  arch/riscv/kernel/sbi.c | 21 ++++++++++++++++++---
> >  1 file changed, 18 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/riscv/kernel/sbi.c b/arch/riscv/kernel/sbi.c
> > index 775d3322b422..5be8f90f325e 100644
> > --- a/arch/riscv/kernel/sbi.c
> > +++ b/arch/riscv/kernel/sbi.c
> > @@ -625,17 +625,32 @@ 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 = -1;
> > +
> > +	if (id < 0)
> > +		id = __sbi_base_ecall(SBI_EXT_BASE_GET_MVENDORID);
> > +
> > +	return id;
> >  }
> >  
> >  long sbi_get_marchid(void)
> >  {
> > -	return __sbi_base_ecall(SBI_EXT_BASE_GET_MARCHID);
> > +	static long id = -1;
> > +
> > +	if (id < 0)
> > +		id = __sbi_base_ecall(SBI_EXT_BASE_GET_MARCHID);
> 
> The marchid register will be negative for commercial architecture ids
> because the MSB must be set.
> 
> > +
> > +	return id;
> >  }
> >  
> >  long sbi_get_mimpid(void)
> >  {
> > -	return __sbi_base_ecall(SBI_EXT_BASE_GET_MIMPID);
> > +	static long id = -1;
> > +
> > +	if (id < 0)
> > +		id = __sbi_base_ecall(SBI_EXT_BASE_GET_MIMPID);
> 
> The spec says this register is "left to the provider" and may be
> left-justified. I don't think we can be sure the MSB will not be set.
> 
> For both cases I guess we need an extra bit to determine if we've cached
> or not
> 
>   static bool cached;
>   static long id;
> 
>   if (!cached) {
>      id = ecall();
>      cached = true;
>   }
> 
>   return id;
> 

thanks for noticing this issue. I did look into the mvendor
csr definition, but then wrongly assumed the other 2 being
similar.

I think for consistency it makes sense to have that extra bit
in all 3 functions too.


Thanks
Heiko

> > +
> > +	return id;
> >  }
> >  
> >  static void sbi_send_cpumask_ipi(const struct cpumask *target)
> 
> Thanks,
> drew
>