linux-kernel - Re: [RFC PATCH 06/28] x86/percpu: Get rid of absolute per-CPU variable placement

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Message-ID: <c4bc67dd-2a42-b318-7830-6741e3579775@gentwo.org>
Date: Wed, 25 Sep 2024 10:56:38 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Ard Biesheuvel <ardb+git@...gle.com>
cc: linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>, 
    x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, 
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Subject: Re: [RFC PATCH 06/28] x86/percpu: Get rid of absolute per-CPU variable
 placement

On Wed, 25 Sep 2024, Ard Biesheuvel wrote:

> The x86_64 approach was needed to accommodate per-task stack protector
> cookies, which used to live at a fixed offset of GS+40, requiring GS to
> be treated as a base register. This is no longer the case, though, and
> so GS can be repurposed as a true per-CPU offset, adopting the same
> strategy as other architectures.
>
> This also removes the need for linker tricks to emit the per-CPU ELF
> segment at a different virtual address. It also means RIP-relative
> per-CPU variables no longer need to be relocated in the opposite
> direction when KASLR is applied, which was necessary because the 0x0
> based per-CPU region remains in place even when the kernel is moved
> around.

Looks like a good cleanup. Hope it does not break anything that relies on
structures %GS points to.

Reviewed-by: Christoph Lameter <cl@...ux.com>