Re: [PATCH v8 1/2] x86/kexec: add a new atomic notifier list for kdump

Gleb Natapov
2012-11-26T17:53:42Z

On Mon, Nov 26, 2012 at 11:43:10AM -0600, Eric W. Biederman wrote:
> Gleb Natapov <gleb@redhat.com> writes:
>
> > On Mon, Nov 26, 2012 at 09:08:54AM -0600, Eric W. Biederman wrote:
> >> Zhang Yanfei <zhangyanfei@cn.fujitsu.com> writes:
> >>
> >> > This patch adds an atomic notifier list named crash_notifier_list.
> >> > Currently, when loading kvm-intel module, a notifier will be registered
> >> > in the list to enable vmcss loaded on all cpus to be VMCLEAR'd if
> >> > needed.
> >>
> >> crash_notifier_list ick gag please no. Effectively this makes the kexec
> >> on panic code path undebuggable.
> >>
> >> Instead we need to use direct function calls to whatever you are doing.
> >>
> > The code walks linked list in kvm-intel module and calls vmclear on
> > whatever it finds there. Since the function have to resides in kvm-intel
> > module it cannot be called directly. Is callback pointer that is set
> > by kvm-intel more acceptable?
>
> Yes a specific callback function is more acceptable. Looking a little
> deeper vmclear_local_loaded_vmcss is not particularly acceptable. It is
> doing a lot of work that is unnecessary to save the virtual registers
> on the kexec on panic path.
>
What work are you referring to in particular that may not be acceptable?

> In fact I wonder if it might not just be easier to call vmcs_clear to a
> fixed per cpu buffer.
>
There may be more than one vmcs loaded on a cpu, hence the list.

> Performing list walking in interrupt context without locking in
> vmclear_local_loaded vmcss looks a bit scary. Not that locking would
> make it any better, as locking would simply add one more way to deadlock
> the system. Only an rcu list walk is at all safe. A list walk that
> modifies the list as vmclear_local_loaded_vmcss does is definitely not safe.
>
The list vmclear_local_loaded walks is per cpu. Zhang's kvm patch
disables kexec callback while list is modified.

--
Gleb.