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Message-ID: <9b152d55-8a1a-75b5-58df-0fe33458107a@zytor.com>
Date: Wed, 19 Jul 2017 19:21:50 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Thomas Garnier <thgarnie@...gle.com>, Brian Gerst <brgerst@...il.com>
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Subject: Re: [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.
-hpa
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