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

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Date:   Wed, 5 Oct 2022 19:07:02 +0200
From:   Andrew Jones <ajones@...tanamicro.com>
To:     Heiko Stuebner <heiko@...ech.de>
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

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;

> +
> +	return id;
>  }
>  
>  static void sbi_send_cpumask_ipi(const struct cpumask *target)
> -- 
> 2.35.1
> 

Thanks,
drew