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Re: [Xen-devel] [v3,11/41] mips: reuse asm-generic/barrier.h

On 01/14/2016 01:34 PM, Paul E. McKenney wrote:
On Thu, Jan 14, 2016 at 12:46:43PM -0800, Leonid Yegoshin wrote:
On 01/14/2016 12:15 PM, Peter Zijlstra wrote:
On Thu, Jan 14, 2016 at 11:42:02AM -0800, Leonid Yegoshin wrote:
An the only point - please use an appropriate SYNC_* barriers instead of
heavy bold hammer. That stuff was design explicitly to support the
requirements of Documentation/memory-barriers.txt
That's madness. That document changes from version to version as to what
we _think_ the actual hardware does. It is _NOT_ a specification.

You cannot design hardware from that. Its incomplete and fails to
specify a bunch of things. It not a mathematically sound definition of a
memory model.

Please stop referring to that document for what a particular barrier
_should_ do.  Explain what MIPS does, so we can attempt to integrate
this knowledge with our knowledge of PPC/ARM/Alpha/x86/etc. and improve
upon our understanding of hardware and improve the Linux memory model.
I am afraid I can't help you here. It is very complicated stuff and
a model is actually doesn't fit your assumptions about CPUs well
without some simplifications which are based on what you want to

I say that SYNC_ACQUIRE/etc follows what you expect for smp_acquire
etc (basing on that document). And at least two CPU models were
tested with my patches (see it in LMO) for that last year and that
instructions are implemented now in engineering kernel.

If you have something else in mind, you can ask me. But I prefer to
do not deviate too much from Documentation/memory-barriers.txt, for
exam - if it asks to have memory barrier somewhere, then I assume
the code should have it, and please - don't ask me a test which
violates the current version of document recommendations.

For a moment I don't see a significant changes in this document for
MIPS Arch at least 1.5 year, and the only significant point is that
MIPS CPU Arch doesn't have yet smp_read_barrier_depends() and
smp_rmb() should be used instead.
Is SYNC_ACQUIRE a memory-barrier instruction that orders prior loads
against later loads and stores?

Yes, it is in MD00087 (table 6.6 of document Ver 6.04) - https://imgtec.com/?do-download=4302

   If so, and if MIPS does not do
ordering based on address and data dependencies, I suggest making
read_barrier_depends() be a SYNC_ACQUIRE rather than SYNC_RMB.

I understood that, after I see the example of using it.
Please consider to add that into Documentation/memory-barriers.txt (it is not easy to find that this barrier is used for shared WRITE basing on shared pointer), it would be helpful.

- Leonid.

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