Re: [PATCH 10/12] kexec: arrange for paddr_vmcoreinfo_note() to return phys_addr_t

From: Baoquan He <bhe@redhat.com>
To: Pratyush Anand <panand@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	devicetree@vger.kernel.org, Tony Luck <tony.luck@intel.com>,
	linux-ia64@vger.kernel.org, Pawel Moll <pawel.moll@arm.com>,
	Ian Campbell <ijc+devicetree@hellion.org.uk>,
	linux-doc@vger.kernel.org, kexec@lists.infradead.org,
	Fenghua Yu <fenghua.yu@intel.com>,
	Haren Myneni <hbabu@us.ibm.com>, Rob Herring <robh+dt@kernel.org>,
	Eric Biederman <ebiederm@xmission.com>,
	Santosh Shilimkar <ssantosh@kernel.org>,
	Russell King <rmk+kernel@arm.linux.org.uk>,
	Jonathan Corbet <corbet@lwn.net>,
	Kumar Gala <galak@codeaurora.org>,
	Vivek Goyal <vgoyal@redhat.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 10/12] kexec: arrange for paddr_vmcoreinfo_note() to return phys_addr_t
Date: Tue, 3 May 2016 17:01:20 +0800	[thread overview]
Message-ID: <20160503090120.GB3287@x1.redhat.com> (raw)
In-Reply-To: <20160503055329.GA13045@dhcppc6.redhat.com>

On 05/03/16 at 11:23am, Pratyush Anand wrote:
> Hi Baoquan,
> 
> On 03/05/2016:12:24:41 PM, Baoquan He wrote:
> > Hi Pratyush,
> > 
> > Could you please help tell why arm PAE kernel can be put above 4G?
> 
> PAE system can have physical addresses above 4G. So, if a CPU is supporting the
> LPAE page table format then we should be able to access physical addresses
> beyond 4G.

OK, after patient explanation from Pratush on irc, I finally understand on
arm LPAE there's PHYS_OFFSET which indicats the physical address of main
memory since arm could map some amount of device memory to cpu address space
below 4G. Then physical ram has to be started from above 4G form cpu
point of view. That's why kernel need be put above 4G. It's fair enough to
fix it in this patch. Certainly if this explanation is also added into
patch log, non arm developer will understand the change better.

Thanks
Baoquan

> 
> > Since the change is related to common code, I am curious about how
> > it's so different with other ARCHs.
> 
> paddr_vmcoreinfo_note() returns a physical address, so returning phys_addr_t
> seems most appropriate. So, kernel variable may land into above 4G locations,
> even with the platform in other architecture (with PAE support), if start of RAM
> is located very high,
> 
> So, in my opinion it would be safer to have these changes for other ARCHs as
> well.
> 
> ~Pratyush
> 
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