Re: [PATCH V11 13/17] RISC-V: paravirt: pvqspinlock: Add SBI implementation

[Leonardo Bras <leobras@redhat.com>]

On Sun, Sep 10, 2023 at 04:29:07AM -0400, guoren@kernel.org wrote:
> From: Guo Ren <guoren@linux.alibaba.com>
> 
> Implement pv_kick with SBI implementation, and add SBI_EXT_PVLOCK
> extension detection.
> 
> Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@kernel.org>
> ---
>  arch/riscv/include/asm/sbi.h           | 6 ++++++
>  arch/riscv/kernel/qspinlock_paravirt.c | 7 ++++++-
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/include/asm/sbi.h b/arch/riscv/include/asm/sbi.h
> index e0233b3d7a5f..3533f8d4f3e2 100644
> --- a/arch/riscv/include/asm/sbi.h
> +++ b/arch/riscv/include/asm/sbi.h
> @@ -30,6 +30,7 @@ enum sbi_ext_id {
>  	SBI_EXT_HSM = 0x48534D,
>  	SBI_EXT_SRST = 0x53525354,
>  	SBI_EXT_PMU = 0x504D55,
> +	SBI_EXT_PVLOCK = 0xAB0401,
>  
>  	/* Experimentals extensions must lie within this range */
>  	SBI_EXT_EXPERIMENTAL_START = 0x08000000,
> @@ -243,6 +244,11 @@ enum sbi_pmu_ctr_type {
>  /* Flags defined for counter stop function */
>  #define SBI_PMU_STOP_FLAG_RESET (1 << 0)
>  
> +/* SBI PVLOCK (kick cpu out of wfi) */
> +enum sbi_ext_pvlock_fid {
> +	SBI_EXT_PVLOCK_KICK_CPU = 0,
> +};
> +
>  #define SBI_SPEC_VERSION_DEFAULT	0x1
>  #define SBI_SPEC_VERSION_MAJOR_SHIFT	24
>  #define SBI_SPEC_VERSION_MAJOR_MASK	0x7f
> diff --git a/arch/riscv/kernel/qspinlock_paravirt.c b/arch/riscv/kernel/qspinlock_paravirt.c
> index a0ad4657f437..571626f350be 100644
> --- a/arch/riscv/kernel/qspinlock_paravirt.c
> +++ b/arch/riscv/kernel/qspinlock_paravirt.c
> @@ -11,6 +11,8 @@
>  
>  void pv_kick(int cpu)
>  {
> +	sbi_ecall(SBI_EXT_PVLOCK, SBI_EXT_PVLOCK_KICK_CPU,
> +		  cpuid_to_hartid_map(cpu), 0, 0, 0, 0, 0);
>  	return;
>  }
>  
> @@ -25,7 +27,7 @@ void pv_wait(u8 *ptr, u8 val)
>  	if (READ_ONCE(*ptr) != val)
>  		goto out;
>  
> -	/* wait_for_interrupt(); */
> +	wait_for_interrupt();
>  out:
>  	local_irq_restore(flags);
>  }
> @@ -62,6 +64,9 @@ void __init pv_qspinlock_init(void)
>  	if(sbi_get_firmware_id() != SBI_EXT_BASE_IMPL_ID_KVM)
>  		return;
>  
> +	if (!sbi_probe_extension(SBI_EXT_PVLOCK))
> +		return;
> +
>  	pr_info("PV qspinlocks enabled\n");
>  	__pv_init_lock_hash();
>  
> -- 
> 2.36.1
> 

IIUC this PVLOCK extension is now a requirement to use pv_qspinlock(), and 
it allows a cpu to use an instruction to wait for interrupt in pv_wait(), 
and kicks it out of this wait using a new sbi_ecall() on pv_kick().

Overall it LGTM, but would be nice to have the reference doc in the commit
msg. I end up inferring some of the inner workings by your implementation, 
which is not ideal for reviewing.

If understanding above is right,
Reviewed-by: Leonardo Bras <leobras@redhat.com>

Thanks!
Leo