linux-kernel - Re: [PATCH 1/1] arm64: fix flush_cache

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Date:	Wed, 25 May 2016 11:36:38 +0800
From:	"Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:	Catalin Marinas <catalin.marinas@....com>
CC:	Mark Rutland <mark.rutland@....com>,
	Tianhong Ding <dingtianhong@...wei.com>,
	Will Deacon <will.deacon@....com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Xinwei Hu <huxinwei@...wei.com>, Zefan Li <lizefan@...wei.com>,
	Hanjun Guo <guohanjun@...wei.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/1] arm64: fix flush_cache_range



On 2016/5/25 9:20, Leizhen (ThunderTown) wrote:
> 
> 
> On 2016/5/24 21:02, Catalin Marinas wrote:
>> On Tue, May 24, 2016 at 08:19:05PM +0800, Leizhen (ThunderTown) wrote:
>>> On 2016/5/24 19:37, Mark Rutland wrote:
>>>> On Tue, May 24, 2016 at 07:16:37PM +0800, Zhen Lei wrote:
>>>>> When we ran mprotect04(a test case in LTP) infinitely, it would always
>>>>> failed after a few seconds. The case can be described briefly that: copy
>>>>> a empty function from code area into a new memory area(created by mmap),
>>>>> then call mprotect to change the protection to PROT_EXEC. The syscall
>>>>> sys_mprotect will finally invoke flush_cache_range, but this function
>>>>> currently only invalid icache, the operation of flush dcache is missed.
>>>>
>>>> In the LTP code I see powerpc-specific D-cache / I-cache synchronisation
>>>> (i.e. d-cache cleaning followed by I-cache invalidation), so there
>>>> appears to be some expectation of userspace maintenance. Hoever, there
>>>> is no such ARM-specific I-cache maintenance.
>>>
>>> But I see some other platforms have D-cache maintenance, like: arch/nios2/mm/cacheflush.c
>>> And according to the name of flush_cache_range, it should do this, I judged. Otherwise,
>>> mprotect04 will be failed on more platforms, it's easy to discover. Only PPC have specific
>>> cache synchronization, maybe it meets some hardware limitation. It's impossible a programmer
>>> fixed a common bug on only one platform but leave others unchanged.
>>
>> flush_cache_range() is primarily used on VIVT caches before changing the
>> mapping and should not really be implemented on arm64. I don't recall
>> why we still have the I-cache invalidation, possibly for the ASID-tagged
>> VIVT I-cache case, though we should have a specific check for this.
>>
>> There are some other cases where flush_cache_range() is called and no
>> D-cache maintenance is necessary on arm64, so I don't want to penalise
>> them by implementing flush_cache_range().
>>
>>>> It looks like the test may be missing I-cache maintenance regardless of
>>>> the semantics of mprotect in this case.
>>>>
>>>> I have not yet devled into flush_cache_range and how it is called.
>>>
>>> SYSCALL_DEFINE3(mprotect ---> mprotect_fixup ---> change_protection ---> change_protection_range --> flush_cache_range
>>
>> The change_protection() shouldn't need to flush the caches in
>> flush_cache_range(). The change_pte_range() function eventually ends up
>> calling set_pte_at() which calls __sync_icache_dcache() if the mapping
>> is executable.
> 
> OK, I see.
> But I'm afraid it entered the "if (pte_present(oldpte))" branch in function change_pte_range.
> Because the test case called mmap to create pte first, then called pte_modify.
> I will check it later.

I have checked that it entered "if (pte_present(oldpte))" branch.

But I don't known why I add flush_icache_range is OK, but add __sync_icache_dcache have no effect.

> 
>>
>> Can you be more specific about the kernel version you are using, its
>> configuration?
>>
> I used the latest mainline kernel version, and built with arch/arm64/configs/defconfig, ran on our D02 board.
> I have attached the testcase, you can simply run: sh test.sh
>