From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gleb Natapov Subject: Re: [PATCH] Don't call cpu_synchronize_state() in apic_init_reset() Date: Thu, 24 Sep 2009 11:03:12 +0300 Message-ID: <20090924080312.GB9832@redhat.com> References: <1253631112-26124-1-git-send-email-gleb@redhat.com> <1253631112-26124-3-git-send-email-gleb@redhat.com> <4AB9E3BC.7060304@redhat.com> <4ABA399F.6030003@web.de> <4ABA3C14.7080803@redhat.com> <4ABA42A1.4010904@web.de> <4ABB2597.7090107@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kiszka , kvm@vger.kernel.org To: Avi Kivity Return-path: Received: from mx1.redhat.com ([209.132.183.28]:38071 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751890AbZIXIDK (ORCPT ); Thu, 24 Sep 2009 04:03:10 -0400 Content-Disposition: inline In-Reply-To: <4ABB2597.7090107@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Thu, Sep 24, 2009 at 10:53:59AM +0300, Avi Kivity wrote: > On 09/23/2009 06:45 PM, Jan Kiszka wrote: > >>Functions calling each other in the same subsystem can rely on callers > >>calling cpu_synchronize_state(). Across subsystems, that's another > >>matter, exported functions should try not to rely on implementation > >>details of their callers. > >> > >>(You might argue that the apic is not separate subsystem wrt an x86 cpu, > >>and I'm not sure I have a counterargument) > >> > >I do accept this argument. It's just that my feeling is that we are > >lacking proper review of the required call sites of cpu_sychronize_state > >and rather put it where some regression popped up (and that only in > >qemu-kvm...). > > That's life... > > >The new rule is: Synchronize the states before accessing registers (or > >in-kernel devices) the first time after a vmexit to user space. > > No, the rule is: synchronize state before accessing registers. > Extra synchronization is cheap, while missing synchronization is > very expensive. > So should we stick cpu_synchronize_state() before each register accesses? I think it is reasonable to omit it if all callers do it already. > >But, > >e.g., I do not see where we do this on CPU reset. > > That's a bug. > Only if kvm support cpus without apic. Otherwise CPU is reset by apic_reset() and cpu_synchronize_state() is called there. -- Gleb.