Re: [PATCH] RISC-V: only flush icache when it has VM_EXEC set

[Palmer Dabbelt]

On Tue, 09 Jan 2024 10:48:59 PST (-0800), cyy@xxxxxxxxxxxx wrote:
> As I-Cache flush on current RISC-V needs to send IPIs to every CPU cores
> in the system is very costly, limiting flush_icache_mm to be called only
> when vma->vm_flags has VM_EXEC can help minimize the frequency of these
> operations. It improves performance and reduces disturbances when
> copy_from_user_page is needed such as profiling with perf.
>
> For I-D coherence concerns, it will not fail if such a page adds VM_EXEC
> flags in the future since we have checked it in the __set_pte_at function.
>
> Signed-off-by: Yangyu Chen <cyy@xxxxxxxxxxxx>
> ---
>  arch/riscv/include/asm/cacheflush.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/include/asm/cacheflush.h b/arch/riscv/include/asm/cacheflush.h
> index 3cb53c4df27c..915f532dc336 100644
> --- a/arch/riscv/include/asm/cacheflush.h
> +++ b/arch/riscv/include/asm/cacheflush.h
> @@ -33,8 +33,11 @@ static inline void flush_dcache_page(struct page *page)
>   * so instead we just flush the whole thing.
>   */
>  #define flush_icache_range(start, end) flush_icache_all()
> -#define flush_icache_user_page(vma, pg, addr, len) \
> -	flush_icache_mm(vma->vm_mm, 0)
> +#define flush_icache_user_page(vma, pg, addr, len)	\
> +do {							\
> +	if (vma->vm_flags & VM_EXEC)			\
> +		flush_icache_mm(vma->vm_mm, 0);		\
> +} while (0)
>
>  #ifdef CONFIG_64BIT
>  #define flush_cache_vmap(start, end)	flush_tlb_kernel_range(start, end)

I'm not super worried about the benchmarks, I think we can just 
open-loop assume this is faster by avoiding the flushes.  I do think we 
need a hook into at least tlb_update_vma_flags(), though, to insert the 
fence.i when upgrading a mapping to include VM_EXEC.