Re: [Xen-devel] Linux questions

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 4/12/07 10:23, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> I have to disagree: At least the uses of barrier() in monotonic_clock() appear
> to be in places where in reality (and from a theoretical standpoint) rmb()
> ought to be used.

We're sync'ing against concurrent updates of a this_cpu variable. We can
only race updates in a local ISR, and hence barrier() suffices.

> But I agree that rmb() (and also wmb()) on x86 doesn't need to be more
> than barrier() (except, as said, in the context of WC memory or non-temporal
> memory accesses) - isn't that exactly what you just recently did in the
> hypervisor?

It is, in response to Intel's new whitepaper on memory ordering guarantees,
and also after seeing similar patches committed in Linux (despite some
protestation!).

Within Linux guests, we will simply follow the barrier definitions for the
tree we are patching. Virtualisation should not affect barrier
implementations.

 -- Keir



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