Re: [linux-fbdev] Re: readl() and friends and eieio on PPC

Richard Henderson [off-list ref] wrote:

No, 2 Cycles to L1 cache.  One cycle to execute the store,
which merely adds an entry to the store buffer.

Yes, of course, silly me.  Same on PPC.

quoted

(Ob. nitpicking: if a wmb takes 12 cycles, how come we can do a wmb
and 8 stores in 10 cycles? :-)

Because it doesn't work like that.  wmb adds a magic token to the
store buffer that prevents write combining and other such hw
optimizations.  Timing

Then why is there such a big performance impact from the wmb's?

Paul.

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