From: "H. Peter Anvin" <hpa@zytor.com>
To: mingo@kernel.org, linux-kernel@vger.kernel.org,
JBeulich@suse.com, tglx@linutronix.de,
linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/boot] x86-64: Use RIP-relative addressing for most per-CPU accesses
Date: Tue, 04 Nov 2014 12:10:43 -0800 [thread overview]
Message-ID: <545932C3.3080907@zytor.com> (raw)
In-Reply-To: <tip-97b67ae559947f1e208439a1bf6a734da3087006@git.kernel.org>
On 11/04/2014 11:45 AM, tip-bot for Jan Beulich wrote:
> Commit-ID: 97b67ae559947f1e208439a1bf6a734da3087006
> Gitweb: http://git.kernel.org/tip/97b67ae559947f1e208439a1bf6a734da3087006
> Author: Jan Beulich <JBeulich@suse.com>
> AuthorDate: Tue, 4 Nov 2014 08:50:48 +0000
> Committer: Thomas Gleixner <tglx@linutronix.de>
> CommitDate: Tue, 4 Nov 2014 20:43:14 +0100
>
> x86-64: Use RIP-relative addressing for most per-CPU accesses
>
> Observing that per-CPU data (in the SMP case) is reachable by
> exploiting 64-bit address wraparound (building on the default kernel
> load address being at 16Mb), the one byte shorter RIP-relative
> addressing form can be used for most per-CPU accesses. The one
> exception are the "stable" reads, where the use of the "P" operand
> modifier prevents the compiler from using RIP-relative addressing, but
> is unavoidable due to the use of the "p" constraint (side note: with
> gcc 4.9.x the intended effect of this isn't being achieved anymore,
> see gcc bug 63637).
>
> With the dependency on the minimum kernel load address, arbitrarily
> low values for CONFIG_PHYSICAL_START are now no longer possible. A
> link time assertion is being added, directing to the need to increase
> that value when it triggers.
>
This description makes very little sense. What matters here is the
virtual address, which is always >= -2 GB.
-hpa
next prev parent reply other threads:[~2014-11-04 20:11 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-11-04 8:50 [PATCH 2/2] x86-64: use RIP-relative addressing for most per-CPU accesses Jan Beulich
2014-11-04 19:45 ` [tip:x86/boot] x86-64: Use " tip-bot for Jan Beulich
2014-11-04 20:10 ` H. Peter Anvin [this message]
2014-11-05 17:04 ` Jan Beulich
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