[PATCH 0/2] kvm: disable virtualization on kdump

Eduardo Habkost ehabkost at redhat.com
Sun Oct 26 17:39:27 EDT 2008

On Sun, Oct 26, 2008 at 05:07:45PM +0200, Avi Kivity wrote:
> Eric W. Biederman wrote:
>>>> Is it possible to disable vmx mode before we enable interrrupts in the
>>>> kdump kernel?
>>> You need IPIs to disable vmx on smp.
>> Thank you.  Reading your description and taking a quick look at
>> the code in hardware disable it does not appear that there is
>> anything needed (other than restricting ourselves it running
>> uniprocessor in the kdump kernel) that needs to happen.
>> Certainly it would be nice to have kvm disabled in hardware,
>> but if you are proposing using the existing hardware disable
>> I must say that the cure looks much worse than the disease.
> Certainly you don't want to issue IPIs when kdump()ing.  It's not  
> unlikely that the other cpus have interrupts permanently disabled.
> (we can use NMI IPIs, but that will likely be messy)

NMI IPIs are already used on x86 native_machine_crash_shutdown(), so
it wouldn't get more messy that it is currently. We just need to add
another bit of code to the code that already runs on an NMI handler.

My question is: is a notifier chain too much complexity for a sensible
piece of code like that? If so, a compile-time hook on that point
would be safer, but then it wouldn't work when KVM is compiled as a
out-of-tree module.

>> It looks like the disable function is all of about 20 assembly
>> instructions so I would not have a problem if he had a
>> little inline function we could call that test to see if
>> vmx is enabled and disable it in the case of kexec on panic.
>> The normal polite shutdown.  That just looks like asking for trouble.
> But what happens when the kdump kernel reboots?  If it is uniprocessor,  
> it will never have a chance to disable vmx on other cpus.  Using acpi  
> reset (now default) works around this on some machines, but not all.

Good point. My problem was a hang when booting the kdump kernel, but it
may also cause problems later, when the kdump kernel reboots.


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