Re: Have any influence on set_memory_** about below patch ??

[Xishi Qiu]

On 2016/1/13 13:02, Xishi Qiu wrote:

> On 2016/1/12 19:15, Mark Rutland wrote:
> 
>> On Tue, Jan 12, 2016 at 09:20:54AM +0800, Xishi Qiu wrote:
>>> On 2016/1/11 21:31, Mark Rutland wrote:
>>>
>>>> Hi,
>>>>
>>>> On Mon, Jan 11, 2016 at 08:59:44PM +0800, zhong jiang wrote:
>>>>>
>>>>> http://www.spinics.net/lists/arm-kernel/msg472090.html
>>>>>
>>>>> Hi, Can I ask you a question? Say, This patch tells that the section spilting
>>>>> and merging wiil produce confilct in the liner mapping area. Based on the
>>>>> situation, Assume that set up page table in 4kb page table way in the liner
>>>>> mapping area, Does the set_memroy_** will work without any conplict??
>>>>
>>>> I'm not sure I understand the question.
>>>>
>>>> I'm also not a fan of responding to off-list queries as information gets
>>>> lost.
>>>>
>>>> Please ask your question on the mailing list. I am more than happy to
>>>> respond there.
>>>>
>>>> Thanks,
>>>> Mark.
>>>>
>>>
>>> Hi Mark,
>>>
>>> In your patch it said "The presence of conflicting TLB entries may result in
>>> a variety of behaviours detrimental to the system " and "but this(break-before-make
>>> approach) cannot work for modifications to the swapper page tables that cover the
>>> kernel text and data."
>>>
>>> I'm not quite understand this, why the direct mapping can't work?
>>
>> The problem is that the TLB hardware can operate asynchronously to the
>> rest of the CPU. At any point in time, for any reason, it can decide to
>> destroy TLB entries, to allocate new ones, or to perform a walk based on
>> the existing contents of the TLB.
>>
>> When the TLB contains conflicting entries, TLB lookups may result in TLB
>> conflict aborts, or may return an "amalgamation" of the conflicting
>> entries (e.g. you could get an erroneous output address).
>>
>> The direct mapping is in active use (and hence live in TLBs). Modifying
>> it without break-before-make (BBM) risks the allocation of conflicting
>> TLB entries. Modifying it with BBM risks unmapping the portion of the
>> kernel performing the modification, resulting in an unrecoverable abort.
>>
>>> flush tlb can't resolve it?
>>
>> Flushing the TLB doesn't help because the page table update, TLB
>> invalidate, and corresponding barrier(s) are separate operations. The
>> TLB can allocate or destroy entries at any point during the sequence.
>>
>> For example, without BBM a page table update would look something like:
>>
>> 1)	str	<newpte>, [<*pte>]
>> 2)	dsb	ish
>> 3)	tlbi	vmalle1is
>> 4)	dsb	ish
>> 5)	isb
>>
>> After step 1, the new pte value may become visible to the TLBs, and the
>> TLBs may allocate a new entry for it. Until step 4 completes, this entry
>> may remain active in the TLB, and may conflict with an existing entry.
>>
>> If that entry covers the kernel text for steps 2-5, executing the
>> sequence may result in an unrecoverable TLB conflict abort, or some
>> other behaviour resulting from an amalgamated TLB, e.g. the I-cache
>> might fetch instructions from the wrong address such that steps 2-5
>> cannot be executed.
>>
>> If the kernel doesn't explicitly access the address covered by that pte,
>> there may still be a problem. The TLB may perform an internal lookup
>> when performing a page table walk, and could then use an erroneous
>> result to continue the walk, resulting in a variety of potential issues
>> (e.g. reading from an MMIO peripheral register).
>>
>> BBM avoids the conflict, but as that would mean kernel text and/or data
>> would be unmapped, you can't execute the code to finish the update.
>>
>>> I find x86 does not have this limit. e.g. set_memory_r*.
>>
>> I don't know much about x86; it's probably worth asking the x86 guys
>> about that. It may be that the x86 architecture requires that a conflict
>> or amalgamation is never visible to software, or it could be that
>> contemporary implementations happen to provide that property.
>>
>> Thanks,
>> Mark.
>>
> 
> Hi Mark,
> 
> If I do like this, does it have the problem too?
> 
> kmalloc a size

exactly is alloc page, the size is aligned with page size.

> no access
> flush tlb
> call set_memory_ro to change the page table flag
> flush tlb
> start access
> 
> Thanks,
> Xishi Qiu 
> 
>> .
>>
> 
>