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Date: Thu, 23 Jan 2014 15:01:09 -0500 (EST)
From: Nicolas Pitre <nico@fluxnic.net>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Wang Nan <wangnan0@huawei.com>, kexec@lists.infradead.org,
        Eric Biederman <ebiederm@xmission.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Geng Hui <hui.geng@huawei.com>, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        stable@vger.kernel.org
Subject: Re: [PATCH 3/3] ARM: allow kernel to be loaded in middle of phymem
In-reply-to: <20140123193137.GA15937@n2100.arm.linux.org.uk>
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On Thu, 23 Jan 2014, Russell King - ARM Linux wrote:

> On Thu, Jan 23, 2014 at 02:15:07PM -0500, Nicolas Pitre wrote:
> > On Wed, 22 Jan 2014, Wang Nan wrote:
> > 
> > > This patch allows the kernel to be loaded at the middle of kernel awared
> > > physical memory. Before this patch, users must use mem= or device tree to cheat
> > > kernel about the start address of physical memory.
> > > 
> > > This feature is useful in some special cases, for example, building a crash
> > > dump kernel. Without it, kernel command line, atag and devicetree must be
> > > adjusted carefully, sometimes is impossible.
> > 
> > With CONFIG_PATCH_PHYS_VIRT the value for PHYS_OFFSET is determined 
> > dynamically by rounding down the kernel image start address to the 
> > previous 16MB boundary.  In the case of a crash kernel, this might be 
> > cleaner to simply readjust __pv_phys_offset during early boot and call 
> > fixup_pv_table(), and then reserve away the memory from the previous 
> > kernel.  That will let you access that memory directly (with gdb for 
> > example) and no pointer address translation will be required.
> 
> We already have support in the kernel to ignore memory below the calculated
> PHYS_OFFSET.  See 571b14375019c3a66ef70d4d4a7083f4238aca30.

Sure.  Anyway what I'm suggesting above  would require that the crash 
kernel be linked at a different virtual address for that to work.  
That's probably more trouble than simply mapping the otherwise still 
unmapped memory from the crashed kernel.


Nicolas
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