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

From: Gleb Natapov <gleb@redhat.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"kexec@lists.infradead.org" <kexec@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Subject: Re: [PATCH v8 1/2] x86/kexec: add a new atomic notifier list for kdump
Date: Mon, 26 Nov 2012 19:53:27 +0200	[thread overview]
Message-ID: <20121126175327.GG12969@redhat.com> (raw)
In-Reply-To: <87fw3wuuoh.fsf@xmission.com>

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.

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