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SubjectRe: [RFC 00/15] x86_64: Optimize percpu accesses
Eric W. Biederman wrote:
> Jeremy Fitzhardinge <jeremy@goop.org> writes:
>
>
>> Eric W. Biederman wrote:
>>
>>> I think we can get away with just simply realloc'ing the percpu area
>>> on each cpu. No fancy table manipulations required. Just update
>>> the base pointer in %gs and in someplace global.
>>>
>>>
>> It's perfectly legitimate to take the address of a percpu variable and store it
>> somewhere. We can't move them around.
>>
>
> Really? I guess there are cases where that makes sense. It is a pretty
> rare case though. Especially when you are not talking about doing it temporarily
> with preemption disabled. There are few enough users of the API I think we can
> certainly explore the cost of forbidding in the general case of storing the
> address of a percpu variable.
>

No, that sounds like a bad idea. For one, how would you enforce it?
How would you check for it? It's one of those things that would mostly
work and then fail very rarely.

Secondly, I depend on it. I register a percpu structure with Xen to
share per-vcpu specific information (interrupt mask, time info, runstate
stats, etc).

J


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