Re: [PATCH] RISC-V: Issue a local tlb flush if possible.

[Paul Walmsley <paul.walmsley@sifive.com>]

Hi Atish,

On Fri, 9 Aug 2019, Atish Patra wrote:

> In RISC-V, tlb flush happens via SBI which is expensive.
> If the target cpumask contains a local hartid, some cost
> can be saved by issuing a local tlb flush as we do that
> in OpenSBI anyways.
> 
> Signed-off-by: Atish Patra <atish.patra@wdc.com>

A few brief comments/questions beyond the ones that others have mentioned:

1. At some point, some RISC-V systems may implement this SBI call in 
hardware, rather than in software.  Then this might wind up becoming a 
de-optimization.  I don't think there's anything to do about that in code 
right now, but it might be worth adding a comment, and thinking about how 
that case might be handled in the platform specification group.

2. If this patch masks or reduces the likelihood of hitting the 
TLB-related crashes that we're seeing, we probably will want to hold off 
on merging this one until we're relatively certain that those other 
problems have been fixed. 



> ---
>  arch/riscv/include/asm/tlbflush.h | 33 +++++++++++++++++++++++++++----
>  1 file changed, 29 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
> index 687dd19735a7..b32ba4fa5888 100644
> --- a/arch/riscv/include/asm/tlbflush.h
> +++ b/arch/riscv/include/asm/tlbflush.h
> @@ -8,6 +8,7 @@
>  #define _ASM_RISCV_TLBFLUSH_H
>  
>  #include <linux/mm_types.h>
> +#include <linux/sched.h>
>  #include <asm/smp.h>
>  
>  /*
> @@ -46,14 +47,38 @@ static inline void remote_sfence_vma(struct cpumask *cmask, unsigned long start,
>  				     unsigned long size)
>  {
>  	struct cpumask hmask;
> +	struct cpumask tmask;
> +	int cpuid = smp_processor_id();
>  
>  	cpumask_clear(&hmask);
> -	riscv_cpuid_to_hartid_mask(cmask, &hmask);
> -	sbi_remote_sfence_vma(hmask.bits, start, size);
> +	cpumask_clear(&tmask);
> +
> +	if (cmask)
> +		cpumask_copy(&tmask, cmask);
> +	else
> +		cpumask_copy(&tmask, cpu_online_mask);
> +
> +	if (cpumask_test_cpu(cpuid, &tmask)) {
> +		/* Save trap cost by issuing a local tlb flush here */
> +		if ((start == 0 && size == -1) || (size > PAGE_SIZE))
> +			local_flush_tlb_all();
> +		else if (size == PAGE_SIZE)
> +			local_flush_tlb_page(start);
> +		cpumask_clear_cpu(cpuid, &tmask);
> +	} else if (cpumask_empty(&tmask)) {
> +		/* cpumask is empty. So just do a local flush */
> +		local_flush_tlb_all();
> +		return;
> +	}
> +
> +	if (!cpumask_empty(&tmask)) {
> +		riscv_cpuid_to_hartid_mask(&tmask, &hmask);
> +		sbi_remote_sfence_vma(hmask.bits, start, size);
> +	}
>  }
>  
> -#define flush_tlb_all() sbi_remote_sfence_vma(NULL, 0, -1)
> -#define flush_tlb_page(vma, addr) flush_tlb_range(vma, addr, 0)
> +#define flush_tlb_all() remote_sfence_vma(NULL, 0, -1)
> +#define flush_tlb_page(vma, addr) flush_tlb_range(vma, addr, (addr) + PAGE_SIZE)
>  #define flush_tlb_range(vma, start, end) \
>  	remote_sfence_vma(mm_cpumask((vma)->vm_mm), start, (end) - (start))
>  #define flush_tlb_mm(mm) \
> -- 
> 2.21.0
> 
> 


- Paul