Re: [PATCH v4 2/2] add icswx support

On Fri, 2011-03-04 at 16:22 -0600, Tseng-Hui (Frank) Lin wrote:

quoted

Well, I don't know how you use icswx on P7+, but on Prism it's
definitely issued directly by userspace.

OK. You've got a point. I wasn't aware of Prism. HFI device driver is
currently the only icswx user on P7. Could you point me to more
information about how Prism uses icswx from user space?

Let's ignore that for now, there's a whole infrastructure for it and it
hasn't been published yet, maybe later this year... We can look at
exposing the feature to userspace then.

quoted

You don't need to. You allocate and initialize the structure, and you
compare & swap the pointer. If somebody beat you, you trash your copy. 

Is atomic_cmpxchg() the one to do the trick?

No, just cmpxchg()

The lazy switching checks the shadow variable first before setting ACOP
register. This saves mtspr() only if the new value is the same as
current. If there are several coprocessors on the system, the ACOP
register may have to be changed frequently. In that case, the lazy
switching will not save time. In extreme case when the ACOP register
needs to be changed every time, it actually slows down the execution by
the additional shadow variable checking.

By how much ? Is it even measurable ?

Cheers,
Ben.