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

[Yangyu Chen]

> On Mar 22, 2024, at 23:50, Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
> 
> On Wed, Mar 20, 2024 at 1:48 AM Palmer Dabbelt <palmer@xxxxxxxxxxx> wrote:
>> 
>> 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.
> 
> I'd say Yangyu is right when he mentions in the commit log: "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.".
> If a region indeed becomes executable, the page table will be modified
> to reflect that and then will pass in __set_pte_at() which, as Yangyu
> mentions, does the right thing. Or am I missing something?
> 

I think so. Unless we have any other way to update PTE rather than
using __set_pte_at, I think it’s safe. I’m too busy these days to
provide a micro enough benchmark.

Thanks,
Yangyu Chen