[RFD] efi assisted kdump

[Petr Tesarik]

On Sat, 24 Jan 2015 21:26:37 +0800
Dave Young <dyoung at redhat.com> wrote:

> Hi,
> 
> Kdump has several limitations currently such as kdump kernel reboot will bypass
> device shutdown path so device drivers should reset during initialization.
> 
>[...]
> 
> ppc64 has a feature "firmware assisted kdump", see below documentation:
> Documentation/powerpc/firmware-assisted-dump.txt

Hi Dave,

while I'm no expert on either UEFI or IBM POWER, I'd like to warn you
that fadump (firmware-assisted dump) on PPC is not quite optimal in its
current form. One of the things that have always irritated me are
excessive RAM requirements.

The problem is that there is only one reboot in fadump - after saving
the dump, the secondary kernel discards the saved area and continues
booting as usual. However, many kernel structuers must be already
allocated at that point, e.g. the memmap array(s), but they are sized
by the total RAM, not the limited amount available to the 2nd kernel.

This issue is not so bad on a typical IBM POWER system, because these
machines tend to be partitioned into many small LPARs, where each has
relatively little RAM. But on a machine with 6TB RAM, you'll have to
reserve approx. 84G merely for the memmap array on the secondary
kernel's boot, which looks just a bit wasteful...

Maybe the two approaches can be combined somehow, so that the
kdump system uses kexec() to transition into the production system, but
that only works if a normal kexec does not have any of the issues
you're trying to address.

Just my two cents,
Petr Tesarik