Re: [PATCH v2 01/11] kexec: introduce kexec_ops struct

I still don't really get why it can't be isolated from dom0, which would make more sense to me, even for a Xen crash.

Andrew Cooper [off-list ref] wrote:

On 22/11/2012 17:47, H. Peter Anvin wrote:

quoted

The other thing that should be considered here is how utterly 
preposterous the notion of doing in-guest crash dumping is in a

system 

quoted

that contains a hypervisor.  The reason for kdump is that on bare

metal 

quoted

there are no other options, but in a hypervisor system the right

thing 

quoted

should be for the hypervisor to do the dump (possibly spawning a

clean 

quoted

I/O domain if the I/O domain is necessary to access the media.)

There is absolutely no reason to have a crashkernel sitting around in

quoted

each guest, consuming memory, and possibly get corrupt.

	-hpa

(Your reply to my email which I can see on the xen devel archive
appears
to have gotten lost somewhere inside the citrix email system, so
apologies for replying out of order)

The kdump kernel loaded by dom0 is for when Xen crashes, not for when
dom0 crashes (although a dom0 crash does admittedly lead to a Xen
crash)

There is no possible way it could be a separate domain; Xen completely
ceases to function as soon as jumps to the entry point of the kdump
image.

~Andrew

-- 
Sent from my mobile phone. Please excuse brevity and lack of formatting.

On 23/11/2012 01:38, H. Peter Anvin wrote:

I still don't really get why it can't be isolated from dom0, which would make more sense to me, even for a Xen crash.

The crash region (as specified by crashkernel= on the Xen command line)
is isolated from dom0.

dom0 (using the kexec utility etc) has the task of locating the Xen
crash notes (using the kexec hypercall interface), constructing a binary
blob containing kernel, initram and gubbins, and asking Xen to put this
blob in the crash region (again, using the kexec hypercall interface).

I do not see how this is very much different from the native case
currently (although please correct me if I am misinformed).  Linux has
extra work to do by populating /proc/iomem with the Xen crash regions
boot (so the kexec utility can reference their physical addresses when
constructing the blob), and should just act as a conduit between the
kexec system call and the kexec hypercall to load the blob.

For within-guest kexec/kdump functionality, I agree that it is barking
mad.  However, we do see cloud operators interested in the idea so VM
administrators can look after their crashes themselves.

~Andrew