From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gleb Natapov 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 Message-ID: <20121126175327.GG12969@redhat.com> References: <50ADE0C2.1000106@cn.fujitsu.com> <50ADE11A.401@cn.fujitsu.com> <87ip8sxuyh.fsf@xmission.com> <20121126172054.GF12969@redhat.com> <87fw3wuuoh.fsf@xmission.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , "kvm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "x86-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org" , "kexec-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org" , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , Zhang Yanfei To: "Eric W. Biederman" Return-path: Content-Disposition: inline In-Reply-To: <87fw3wuuoh.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kexec-bounces-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org Errors-To: kexec-bounces+glkk-kexec=m.gmane.org-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org List-Id: kvm.vger.kernel.org On Mon, Nov 26, 2012 at 11:43:10AM -0600, Eric W. Biederman wrote: > Gleb Natapov writes: > > > On Mon, Nov 26, 2012 at 09:08:54AM -0600, Eric W. Biederman wrote: > >> Zhang Yanfei 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.