Re: [PATCH] kdump, x86: report actual value of phys_base in VMCOREINFO

From: Petr Tesarik <ptesarik@suse.cz>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: "HATAYAMA, Daisuke" <d.hatayama@jp.fujitsu.com>,
	kumagai-atsushi@mxc.nes.nec.co.jp, anderson@redhat.com,
	kexec@lists.infradead.org, ebiederm@xmission.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kdump, x86: report actual value of phys_base in VMCOREINFO
Date: Thu, 13 Nov 2014 15:48:10 +0100	[thread overview]
Message-ID: <20141113154810.2ee36e72@hananiah.suse.cz> (raw)
In-Reply-To: <20141113142548.GA1978@redhat.com>

On Thu, 13 Nov 2014 09:25:48 -0500
Vivek Goyal <vgoyal@redhat.com> wrote:

> On Thu, Nov 13, 2014 at 05:30:21PM +0900, HATAYAMA, Daisuke wrote:
> > 
> > (2014/11/13 17:06), Petr Tesarik wrote:
> > >On Thu, 13 Nov 2014 09:17:09 +0900 (JST)
> > >HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com> wrote:
> > >
> > >>From: Vivek Goyal <vgoyal@redhat.com>
> > >>Subject: Re: [PATCH] kdump, x86: report actual value of phys_base in VMCOREINFO
> > >>Date: Wed, 12 Nov 2014 17:12:05 -0500
> > >>
> > >>>On Wed, Nov 12, 2014 at 03:40:42PM +0900, HATAYAMA Daisuke wrote:
> > >>>>Currently, VMCOREINFO note information reports the virtual address of
> > >>>>phys_base that is assigned to symbol phys_base. But this doesn't make
> > >>>>sense because to refer to value of the phys_base, it's necessary to
> > >>>>get the value of phys_base itself we are now about to refer to.
> > >>>>
> > >>>
> > >>>Hi Hatayama,
> > >>>
> > >>>/proc/vmcore ELF headers have virtual address information and using
> > >>>that you should be able to read actual value of phys_base. gdb deals
> > >>>with virtual addresses all the time and can read value of any symbol
> > >>>using those headers.
> > >>>
> > >>>So I am not sure what's the need for exporting actual value of
> > >>>phys_base.
> > >>>
> > >>
> > >>Sorry, my logic in the patch description was wrong. For /proc/vmcore,
> > >>there's enough information for makedumpdile to get phys_base. It's
> > >>correct. The problem here is that other crash dump mechanisms that run
> > >>outside Linux kernel independently don't have information to get
> > >>phys_base.
> > >
> > >Yes, but these mechanisms won't be able to read VMCOREINFO either, will
> > >they?
> > >
> > 
> > I don't intend such sophisticated function only by VMCOREINFO.
> > Search vmcore for VMCOREINFO using strings + grep before opening it by crash.
> > I intend that only here.
> 
> I think this is very crude and not proper way to get to vmcoreinfo.

Same here. If VMCOREINFO must be locatable without communicating any
information to the hypervisor, then I would rather go for something
similar to what s390(x) folks do - a well-known location in physical
memory that contains a pointer to a checksummed OS info structure,
which in turn contains the VMCOREINFO pointers.

I'm a bit surprised such mechanism is not needed by Fujitsu SADUMP.
Or is that part of the current plan, Daisuke?

Petr T