From mboxrd@z Thu Jan 1 00:00:00 1970 From: Glauber Costa Subject: Re: [patch 09/18] KVM: x86: introduce facility to support vsyscall pvclock, via MSR Date: Tue, 30 Oct 2012 11:41:44 +0400 Message-ID: <508F84B8.5030908@parallels.com> References: <20121024131340.742340256@redhat.com> <20121024131621.707068244@redhat.com> <508E9697.2000003@parallels.com> <508EC089.5030409@goop.org> <20121029184015.GA30422@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Cc: Jeremy Fitzhardinge , , , , , , To: Marcelo Tosatti Return-path: Received: from mx2.parallels.com ([64.131.90.16]:37050 "EHLO mx2.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755548Ab2J3Hly (ORCPT ); Tue, 30 Oct 2012 03:41:54 -0400 In-Reply-To: <20121029184015.GA30422@amt.cnet> Sender: kvm-owner@vger.kernel.org List-ID: On 10/29/2012 10:40 PM, Marcelo Tosatti wrote: > On Mon, Oct 29, 2012 at 10:44:41AM -0700, Jeremy Fitzhardinge wrote: >> 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 > > It is possible to share the data for KVM in theory, but: > > - It is a small amount of memory. > - It requires aligning to page size (the in-kernel percpu array > is currently cacheline aligned). > - It is possible to modify flags separately for userspace/kernelspace, > if desired. > > This justifies the duplication IMO (code is simple and clean). > Duplicating is indeed no the end of the world. But one note: * If it is page-size aligned, it is automatically cacheline aligned. Since we have to export the user page *anyway*, this is a non-issue. That said, duplicating instead of integrating, which is technically possible, is a design decision, and it needs to be documented somewhere aside from this mail thread.