Re: [Xen-devel] Re: [PATCH]: kexec: framework and i386

[Hirokazu Takahashi <taka@xxxxxxxxxxxxx>]

Hi,

> > > In summary, for kdump "linux crashdump=64M@16M"
> > > becomes "xen kdump_megabytes=64 kdump_megabytes_base=16"
> >
> > Please can you explain a bit about how kdump works, and why a physically
> > contiguous region with known base address is required. What actually
> > gets written out in the crash dump and in what format?
> 
> The reserved region is the memory space for the "dump kernel".  I believe the 
> base address has to correspond to the base address compiled into the dump 
> kernel - since we don't want the dump kernel to try to own all of memory.  
> It's native Linux, so it likes to run in contiguous memory.

The current implementation of linux kernel for x86 requires:
  1. Memory for the kernel have to be physically contiguous.
  2. The physical memory have to be mapped to specific virtual space.
     It assumes:
         virtual address for the kernel == physical address | 0x80000000.

> When a panic occurs, Linux kexec jumps into the preloaded kdump kernel (if 
> any).  This kernel then reinitiases the hardware, using its own device 
> drivers and uses these to write out the dump to disk.  ISTR that the dump 
> format is currently ELF, although I remember some talk on the Fastboot ML 
> about adding some extra headers to make OS debugging easier.
> 
> It's a nice solution because you don't rely on the hosed kernel to do the 
> dump 
> for you, and you don't disturb its state in the process.  It also makes it 
> easy to do things like dumping to network devices, etc.
> 
> In our case it has the added bonus that on dom0 *or* a Xen crash it ought to 
> be possible to kexec into a native Linux kernel which could dump (possibly 
> some configurable combination of) Xen itself, dom0, and all the other 
> domains.  Admittedly hypervisor crashes / hangs are rare, but it might aid 
> debugging to be able to get a reliable dump of a crashed / hung Xen.
> 
> This would also integrate with the Linux dump infrastructure, which would be 
> useful to have.

After the dump, I think you will be able to kexec a new Xen and dom0 to
restart the box automatically.

> > Tim Deegan submitted a patch to add support for multiboot images (such
> > as Xen) to kexec a couple of years ago, and I believe it has been part
> > of the standard package for some time.
> 
> It was in there last time I looked at the source code...  I've never actually 
> used it though, so in principle I guess it could have rotted.  Or there could 
> just be something weird happenning.
> 
> Cheers,
> Mark

Thanks,
Hirokazu Takahashi.

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