From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [RFC] Unify KVM kernel-space and user-space code into a single project Date: Mon, 22 Mar 2010 21:46:50 +0100 Message-ID: <20100322204650.GD18126@elte.hu> References: <20100322143212.GE14201@elte.hu> <4BA7821C.7090900@codemonkey.ws> <20100322155505.GA18796@elte.hu> <4BA796DF.7090005@redhat.com> <20100322165107.GD18796@elte.hu> <4BA7A406.9050203@redhat.com> <20100322173400.GB15795@elte.hu> <4BA7B9E0.5080009@codemonkey.ws> <20100322192739.GE21919@elte.hu> <4BA7C96D.2020702@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Anthony Liguori , Pekka Enberg , "Zhang, Yanmin" , Peter Zijlstra , Sheng Yang , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Marcelo Tosatti , oerg Roedel , Jes Sorensen , Gleb Natapov , Zachary Amsden , ziteng.huang@intel.com, Arnaldo Carvalho de Melo , Fr?d?ric Weisbecker , Gregory Haskins To: Avi Kivity Return-path: Content-Disposition: inline In-Reply-To: <4BA7C96D.2020702@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org * Avi Kivity wrote: > On 03/22/2010 09:27 PM, Ingo Molnar wrote: > > > >> If your position basically boils down to, we can't trust userspace > >> and we can always trust the kernel, I want to eliminate any > >> userspace path, then I can't really help you out. > > > > Why would you want to 'help me out'? I can tell a good solution from a bad > > one just fine. > > You are basically making a kernel implementation a requirement, instead of > something that follows from the requirement. No, i'm not. > > You should instead read the long list of disadvantages above, invert them > > and list then as advantages for the kernel-based vcpu enumeration > > solution, apply common sense and go admit to yourself that indeed in this > > situation a kernel provided enumeration of vcpu contexts is the most > > robust solution. > > Having qemu enumerate guests one way or another is not a good idea IMO since > it is focused on one guest and doesn't have a system-wide entity. A > userspace system-wide entity will work just as well as kernel > implementation, without its disadvantages. A system-wide user-space entity only solves one problem out of the 4 i listed, still leaving the other 3: - Those special files can get corrupted, mis-setup, get out of sync, or can be hard to discover. - Apps might start KVM vcpu instances without adhering to the system-wide access method. - There is no guarantee for the system-wide process to reply to a request - while the kernel can always guarantee an enumeration result. I dont want 'perf kvm' to hang or misbehave just because the system-wide entity has hung. Really, i think i have to give up and not try to convince you guys about this anymore - i dont think you are arguing constructively anymore and i dont want yet another pointless flamewar about this. Please consider 'perf kvm' scrapped indefinitely, due to lack of robust KVM instrumentation features: due to lack of robust+universal vcpu/guest enumeration and due to lack of robust+universal symbol access on the KVM side. It was a really promising feature IMO and i invested two days of arguments into it trying to find a workable solution, but it was not to be. Thanks, Ingo