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

[Mark Rutland]

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.