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Re: [Xen-devel] [RFC 16/22] x86/percpu: Adapt percpu for PIE support

On 07/19/17 16:33, H. Peter Anvin wrote:
>> I agree that it is odd but that's how the compiler generates code. I
>> will re-explore PIC options with mcmodel=small or medium, as mentioned
>> on other threads.
> Why should the way compiler generates code affect the way we do things
> in assembly?
> That being said, the compiler now has support for generating this kind
> of code explicitly via the __seg_gs pointer modifier.  That should let
> us drop the __percpu_prefix and just use variables directly.  I suspect
> we want to declare percpu variables as "volatile __seg_gs" to account
> for the possibility of CPU switches.
> Older compilers won't be able to work with this, of course, but I think
> that it is acceptable for those older compilers to not be able to
> support PIE.

Grump.  It turns out that the compiler doesn't do the right thing for
symbols marked with the __seg_[fg]s markers.  __thread does the right
thing, but __thread a) has %fs: hard-coded, still, and b) I believe can
still cache %seg:0 arbitrarily long.


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