From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeremy Fitzhardinge Subject: Re: [patch 09/18] KVM: x86: introduce facility to support vsyscall pvclock, via MSR Date: Mon, 29 Oct 2012 10:44:41 -0700 Message-ID: <508EC089.5030409@goop.org> References: <20121024131340.742340256@redhat.com> <20121024131621.707068244@redhat.com> <508E9697.2000003@parallels.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , kvm@vger.kernel.org, johnstul@us.ibm.com, zamsden@gmail.com, gleb@redhat.com, avi@redhat.com, pbonzini@redhat.com To: Glauber Costa Return-path: Received: from claw.goop.org ([74.207.240.146]:34864 "EHLO claw.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752741Ab2J2Rve (ORCPT ); Mon, 29 Oct 2012 13:51:34 -0400 In-Reply-To: <508E9697.2000003@parallels.com> Sender: kvm-owner@vger.kernel.org List-ID: On 10/29/2012 07:45 AM, Glauber Costa wrote: > On 10/24/2012 05:13 PM, Marcelo Tosatti wrote: >> Allow a guest to register a second location for the VCPU time info >> >> structure for each vcpu (as described by MSR_KVM_SYSTEM_TIME_NEW). >> This is intended to allow the guest kernel to map this information >> into a usermode accessible page, so that usermode can efficiently >> calculate system time from the TSC without having to make a syscall. >> >> Signed-off-by: Marcelo Tosatti > Can you please be a bit more specific about why we need this? Why does > the host need to provide us with two pages with the exact same data? Why > can't just do it with mapping tricks in the guest? In Xen the pvclock structure is embedded within a pile of other stuff that shouldn't be mapped into guest memory, so providing for a second location allows it to be placed whereever is convenient for the guest. That's a restriction of the Xen ABI, but I don't know if it affects KVM. J