linux-kernel - Re: [Xen-devel] [PATCH] xen: Add support for dom0 with Linux kernel 3.19 and newer

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Date:	Fri, 22 Jan 2016 10:03:34 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Daniel Kiper <daniel.kiper@...cle.com>, <crash-utility@...hat.com>,
	<kexec@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
	<xen-devel@...ts.xenproject.org>
CC:	<jgross@...e.com>, <olaf@...fle.de>, <keir@....org>,
	<jbeulich@...e.com>, <andrew.cooper3@...rix.com>,
	<ptesarik@...e.cz>, <dslutz@...izon.com>, <anderson@...hat.com>,
	<david.vrabel@...rix.com>, <dmair@...e.com>,
	<boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [PATCH] xen: Add support for dom0 with Linux kernel
 3.19 and newer

On 21/01/16 20:13, Daniel Kiper wrote:
> Linux kernel commit 054954eb051f35e74b75a566a96fe756015352c8
> (xen: switch to linear virtual mapped sparse p2m list), which
> appeared in 3.19, introduced linear virtual mapped sparse p2m
> list. If readmem() reads p2m then it access this list using
> physical addresses. Sadly, VMA to physical address translation
> in crash requires access to p2m list. This means that we have
> a chicken and egg problem. In general this issue must be solved
> by introducing some changes in libxl, Linux kernel and crash
> (I have added this task to my long TODO list). However, in dom0
> case we can use crash_xen_info_t.dom0_pfn_to_mfn_frame_list_list
> which is available out of the box. So, let's use it and make
> at least some users happy.

I'm confused.  How does a virtual address to (pseudo-)physical address
lookup require access to the p2m?  Surely this is a walk of the page
tables followed by a M2P lookup on the MFN in the L1 PTE?

David