On Tue, 27 Jan 2015 14:21:41 +0800 Dave Young <dyoung at redhat.com> wrote: > Hi, Petr > > On 01/24/15 at 05:03pm, Petr Tesarik wrote: > > 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. > > Thanks for the comment. > > I'm not sure I understand the RAM requirements you mentioned. I think you > are worrying about freeing oldmem to be used by 2nd kernel. > > But we do not need to do same as power we can just reboot another time > as long as vmcore is saved. The main advantage is the capture kernel can > boot with all devices being reset. Yes, that's true. If you're fine with rebooting the machine twice (once for the dump-taking and once for bringing up the normal system), then there's no concern. The IBM folks apparently wanted to save some downtime associated with going through the boot-up sequence. In fact, you could kexec back into the normal system instead of rebooting, if kexec can be used. All I wanted to say is that fadump has its own issues, and we'd better not repeat them with UEFI-based dumps. ;-) Petr Tesarik
