Re: [PATCH] RISC-V: Don't use IPIs in flush_icache_all() when patching text

[Alexandre Ghiti]

Hi Anup,

On 05/02/2024 05:29, Anup Patel wrote:
> If some of the HARTs are parked by stop machine then IPI-based
> flushing in flush_icache_all() will hang. This hang is observed
> when text patching is invoked by various debug and BPF features.
>
> To avoid this hang, we force use of SBI-based icache flushing
> when patching text.
>
> Fixes: 627922843235 ("RISC-V: Use IPIs for remote icache flush when possible")
> Reported-by: Bjorn Topel <bjorn@xxxxxxxxxx>
> Closes: https://gist.github.com/bjoto/04a580568378f3b5483af07cd9d22501
> Signed-off-by: Anup Patel <apatel@xxxxxxxxxxxxxxxx>
> ---
>   arch/riscv/include/asm/cacheflush.h | 7 ++++---
>   arch/riscv/kernel/hibernate.c       | 2 +-
>   arch/riscv/kernel/patch.c           | 4 ++--
>   arch/riscv/mm/cacheflush.c          | 7 ++++---
>   4 files changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/arch/riscv/include/asm/cacheflush.h b/arch/riscv/include/asm/cacheflush.h
> index a129dac4521d..561e079f34af 100644
> --- a/arch/riscv/include/asm/cacheflush.h
> +++ b/arch/riscv/include/asm/cacheflush.h
> @@ -32,7 +32,8 @@ static inline void flush_dcache_page(struct page *page)
>    * RISC-V doesn't have an instruction to flush parts of the instruction cache,
>    * so instead we just flush the whole thing.
>    */
> -#define flush_icache_range(start, end) flush_icache_all()
> +#define flush_icache_range(start, end) flush_icache_all(true)
> +#define flush_icache_patch_range(start, end) flush_icache_all(false)
>   #define flush_icache_user_page(vma, pg, addr, len) \
>   	flush_icache_mm(vma->vm_mm, 0)
>   
> @@ -43,12 +44,12 @@ static inline void flush_dcache_page(struct page *page)
>   
>   #ifndef CONFIG_SMP
>   
> -#define flush_icache_all() local_flush_icache_all()
> +#define flush_icache_all(want_ipi) local_flush_icache_all()
>   #define flush_icache_mm(mm, local) flush_icache_all()
>   
>   #else /* CONFIG_SMP */
>   
> -void flush_icache_all(void);
> +void flush_icache_all(bool want_ipi);
>   void flush_icache_mm(struct mm_struct *mm, bool local);
>   
>   #endif /* CONFIG_SMP */
> diff --git a/arch/riscv/kernel/hibernate.c b/arch/riscv/kernel/hibernate.c
> index 671b686c0158..388f10e187ba 100644
> --- a/arch/riscv/kernel/hibernate.c
> +++ b/arch/riscv/kernel/hibernate.c
> @@ -153,7 +153,7 @@ int swsusp_arch_suspend(void)
>   	} else {
>   		suspend_restore_csrs(hibernate_cpu_context);
>   		flush_tlb_all();
> -		flush_icache_all();
> +		flush_icache_all(true);
>   
>   		/*
>   		 * Tell the hibernation core that we've just restored the memory.
> diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
> index 37e87fdcf6a0..721e144a7847 100644
> --- a/arch/riscv/kernel/patch.c
> +++ b/arch/riscv/kernel/patch.c
> @@ -182,7 +182,7 @@ int patch_text_set_nosync(void *addr, u8 c, size_t len)
>   	ret = patch_insn_set(tp, c, len);
>   
>   	if (!ret)
> -		flush_icache_range((uintptr_t)tp, (uintptr_t)tp + len);
> +		flush_icache_patch_range((uintptr_t)tp, (uintptr_t)tp + len);
>   
>   	return ret;
>   }
> @@ -217,7 +217,7 @@ int patch_text_nosync(void *addr, const void *insns, size_t len)
>   	ret = patch_insn_write(tp, insns, len);
>   
>   	if (!ret)
> -		flush_icache_range((uintptr_t) tp, (uintptr_t) tp + len);
> +		flush_icache_patch_range((uintptr_t) tp, (uintptr_t) tp + len);
>   
>   	return ret;
>   }
> diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
> index 55a34f2020a8..03cd3d4831ef 100644
> --- a/arch/riscv/mm/cacheflush.c
> +++ b/arch/riscv/mm/cacheflush.c
> @@ -17,11 +17,12 @@ static void ipi_remote_fence_i(void *info)
>   	return local_flush_icache_all();
>   }
>   
> -void flush_icache_all(void)
> +void flush_icache_all(bool want_ipi)
>   {
>   	local_flush_icache_all();
>   
> -	if (IS_ENABLED(CONFIG_RISCV_SBI) && !riscv_use_ipi_for_rfence())
> +	if (IS_ENABLED(CONFIG_RISCV_SBI) &&
> +	    (!want_ipi || !riscv_use_ipi_for_rfence()))
>   		sbi_remote_fence_i(NULL);
>   	else
>   		on_each_cpu(ipi_remote_fence_i, NULL, 1);
> @@ -87,7 +88,7 @@ void flush_icache_pte(pte_t pte)
>   	struct folio *folio = page_folio(pte_page(pte));
>   
>   	if (!test_bit(PG_dcache_clean, &folio->flags)) {
> -		flush_icache_all();
> +		flush_icache_all(true);
>   		set_bit(PG_dcache_clean, &folio->flags);
>   	}
>   }


Since patch_text_cb() is run on all cpus, couldn't we completely avoid 
any remote icache flush by slightly changing patch_text_cb() instead as 
follows?

diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
index 37e87fdcf6a0..075c376ed528 100644
--- a/arch/riscv/kernel/patch.c
+++ b/arch/riscv/kernel/patch.c
@@ -232,8 +232,8 @@ static int patch_text_cb(void *data)
        if (atomic_inc_return(&patch->cpu_count) == num_online_cpus()) {
                for (i = 0; ret == 0 && i < patch->ninsns; i++) {
                        len = GET_INSN_LENGTH(patch->insns[i]);
-                       ret = patch_text_nosync(patch->addr + i * len,
- &patch->insns[i], len);
+                       ret = patch_insn_write((u32 *)(patch->addr + i * 
len),
+ &patch->insns[i], len);
                }
                atomic_inc(&patch->cpu_count);
        } else {
@@ -242,6 +242,8 @@ static int patch_text_cb(void *data)
                smp_mb();
        }

+       local_flush_icache_all();
+
        return ret;
 }
 NOKPROBE_SYMBOL(patch_text_cb);