[Question] A question about arm64 pte

[Yisheng Xie]

On 2017/1/16 20:57, Steve Capper wrote:
> On Mon, Jan 16, 2017 at 08:39:56PM +0800, Yisheng Xie wrote:
>> hi Catalin,
>> Thank you so much for you reply.
>>
>> On 2017/1/16 19:56, Catalin Marinas wrote:
>>> On Mon, Jan 16, 2017 at 06:08:47PM +0800, Yisheng Xie wrote:
>>>> I have question about arm64 pte.
>>>
>>> I assume the context is ARMv8.0 (without hardware DBM support).
>>
>> Yes.
>>>
>>>> For arm64, PTE_WRITE（== PTE_DBM） is to mark whether the page is writable,
>>>> and PTE_DIRTY is to mark whether the page is dirty.
>>>> However, PTE_RDONLY is only cleared when both PTE_WRITE and PTE_DIRTY are set.
>>>
>>> That's what set_pte_at() does.
>>>
>>
>> So if we mmap a memory region use /dev/mem like:
>>    fildes = open("/dev/mem", O_RDWR | O_CREAT, 0777);
>>    addr = mmap(NULL, LEN, PROT_READ | PROT_WRITE, MAP_SHARED, fildes, offset);
>>
>> The PTE_RDONLY will be set， Right ?
>> However，when use memset to write the region it still works well, and the bit PTE_RDONLY is also cleared.
>> Is there anywhere clear the PTE_RDONLY before write that page ?
>>
> 
> Hi Yisheng,
> Out of interest, why is /dev/mem being accessed directly from userspace?
hi Steve,
Thank you for your reply.
We just want to access some reserved memory region.

Thanks
Yisheng Xie.

> 
> The case above will have subtley different logic (mmap_mem will affect
> how things are actually mapped); which I'm trying to understand...
> 
> Cheers,
>