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Message-ID: <CAJcbSZFjiqPoOUDFSU_WhneEJCEhrAe6nV4tRGOpRg7hsOk1-A@mail.gmail.com>
Date: Wed, 19 Jul 2017 15:44:48 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Brian Gerst <brgerst@...il.com>, Herbert Xu <herbert@...dor.apana.org.au>,
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Subject: Re: [RFC 06/22] kvm: Adapt assembly for PIE support
On Wed, Jul 19, 2017 at 3:27 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 07/19/17 08:40, Thomas Garnier wrote:
>>>
>>> This doesn't look right. It's accessing a per-cpu variable. The
>>> per-cpu section is an absolute, zero-based section and not subject to
>>> relocation.
>>
>> PIE does not respect the zero-based section, it tries to have
>> everything relative. Patch 16/22 also adapt per-cpu to work with PIE
>> (while keeping the zero absolute design by default).
>>
>
> This is silly. The right thing is for PIE is to be explicitly absolute,
> without (%rip). The use of (%rip) memory references for percpu is just
> an optimization.
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.
>
> -hpa
>
--
Thomas
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