linux-kernel - Re: [PATCH v3 02/31] arm64: Kernel booting and initialisation

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Message-ID: <20120910160856.GG27042@arm.com>
Date:	Mon, 10 Sep 2012 17:08:56 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>
Cc:	Nicolas Pitre <nico@...xnic.net>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 02/31] arm64: Kernel booting and initialisation

On Mon, Sep 10, 2012 at 04:21:02PM +0100, Jean-Christophe PLAGNIOL-VILLARD wrote:
> On 13:51 Mon 10 Sep     , Catalin Marinas wrote:
> > On Mon, Sep 10, 2012 at 06:53:39AM +0100, Jean-Christophe PLAGNIOL-VILLARD wrote:
> > > On 19:29 Sun 09 Sep     , Nicolas Pitre wrote:
> > > > On Sun, 9 Sep 2012, Jean-Christophe PLAGNIOL-VILLARD wrote:
> > > > > On 17:26 Fri 07 Sep     , Catalin Marinas wrote:
> > > > > > +The image must be placed at the specified offset (currently 0x80000)
> > > > > > +from the start of the system RAM and called there. The start of the
> > > > > > +system RAM must be aligned to 2MB.
> > > > > can we drop this
> > > > 
> > > > Drop what?
> > > > And why?
> > > This contrain the must be loadable at any address
> > 
> > You can't easily load the kernel image at any address, unless it can
> > relocate itself and you have a way to specify PHYS_OFFSET. We don't want
> > a compile-time PHYS_OFFSET, the kernel detects it at boot time based on
> > the load address.
> so NACK kexec and other boot loaders require it

Just in case it wasn't clear, the kernel can be loaded at any address in
RAM (and the RAM can start at any sane address). The way the kernel
calculates PHYS_OFFSET is (load address - TEXT_OFFSET) unless we pass it
by other means (none currently specified). For a kdump kernel, it just
assumes that its PHYS_OFFSET is higher but it can ioremap the crashed
kernel memory.

-- 
Catalin
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