----- Original Message -----

From: "Kirill A. Shutemov" <redacted>
To: "Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>
Cc: "Matthew Wilcox" <redacted>, "Ross Zwisler" <redacted>, "lttng-dev"
[off-list ref], linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Sent: Tuesday, October 28, 2014 6:54:58 AM
Subject: Re: Progress on system crash traces with LTTng using DAX and pmem

On Sat, Oct 25, 2014 at 12:51:25PM +0000, Mathieu Desnoyers wrote:

quoted

FYI, the main reason why my customer wants to go with a
"trace into memory that survives soft reboot" approach
rather than to use things like kexec/kdump is that they
care about the amount of time it takes to reboot their
machines. They want a solution where they can extract the
detailed crash data after reboot, after the machine is
back online, rather than requiring a few minutes of offline
time to extract the crash details.

IIRC, on x86 there's no guarantee that your memory content will be
preserved over reboot. BIOS is free to mess with it.

Hi Kirill,

This is a good point,

There are a few more aspects to consider here:

- Other architectures appear to have different guarantees, for
  instance ARM which, AFAIK, does not reset memory on soft
  reboot (well at least for my customer's boards). So I guess
  if x86 wants to be competitive, it would be good for them to
  offer a similar feature,

- Already having a subset of machines supporting this is useful,
  e.g. storing trace buffers and recovering them after a crash,

- Since we are in a world of dynamically upgradable BIOS, perhaps
  if we can show that there is value in having a BIOS option to
  specify a memory range that should not be reset on soft reboot,
  BIOS vendors might be inclined to include an option for it,

- Perhaps UEFI BIOS already have some way of specifying that a
  memory range should not be reset on soft reboot ?

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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