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Message-Id: <1159164232.26986.59.camel@localhost.localdomain>
Date: Mon, 25 Sep 2006 16:03:52 +1000
From: Rusty Russell <rusty@...tcorp.com.au>
To: Jeremy Fitzhardinge <jeremy@...p.org>
Cc: Andi Kleen <ak@....de>,
lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
virtualization <virtualization@...ts.osdl.org>
Subject: Re: [PATCH 5/7] Use %gs for per-cpu sections in kernel
On Sun, 2006-09-24 at 22:25 -0700, Jeremy Fitzhardinge wrote:
> The %gs:per_cpu__foo addressing mode still calculates
> 0xbcef00+0xc0433800, which is still a subtraction. My essential point
> is that *all* kernel addresses (=kernel symbols) are negative, so using
> them as an offset from a segment base (any segment base) is a
> subtraction, which requires a 4G limit.
I don't think so. There's *never* address subtraction, there's
sometimes 32 bit wrap (glibc uses this to effect subtraction, sure).
But there's no wrap here.
To test, I changed the following:
--- smpboot.c.~8~ 2006-09-25 15:51:50.000000000 +1000
+++ smpboot.c 2006-09-25 16:00:36.000000000 +1000
@@ -926,8 +926,9 @@
unsigned long per_cpu_off)
{
unsigned limit, flags;
+ extern char __per_cpu_end[];
- limit = (1 << 20);
+ limit = PAGE_ALIGN((long)__per_cpu_end) >> PAGE_SHIFT;
flags = 0x8; /* 4k granularity */
/* present read-write data segment */
Works fine...
Hope that clarifies!
Rusty.
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