From mboxrd@z Thu Jan 1 00:00:00 1970 From: Boris Ostrovsky Subject: Re: [PATCH 01/13] x86/time.c: Use system time to calculate elapsed_nsec in tsc_get_info() Date: Fri, 9 Oct 2015 11:37:06 -0400 Message-ID: <5617DF22.8070602@oracle.com> References: <1443424438-13404-1-git-send-email-haozhong.zhang@intel.com> <1443424438-13404-2-git-send-email-haozhong.zhang@intel.com> <5617801402000078000A992C@prv-mh.provo.novell.com> <5617C410.2090208@oracle.com> <5617EDDD02000078000A9C65@prv-mh.provo.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <5617EDDD02000078000A9C65@prv-mh.provo.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich Cc: Haozhong Zhang , Kevin Tian , Keir Fraser , Suravee Suthikulpanit , Andrew Cooper , xen-devel@lists.xen.org, Aravind Gopalakrishnan , Jun Nakajima List-Id: xen-devel@lists.xenproject.org On 10/09/2015 10:39 AM, Jan Beulich wrote: >>>> On 09.10.15 at 15:41, wrote: >> On 10/09/2015 02:51 AM, Jan Beulich wrote: >>>>>> On 28.09.15 at 09:13, wrote: >>>> When the TSC mode of a domain is TSC_MODE_DEFAULT and no TSC emulation >>>> is used, the existing tsc_get_info() calculates elapsed_nsec by scaling >>>> the host TSC with a ratio between guest TSC rate and >>>> nanoseconds. However, the result will be incorrect if the guest TSC rate >>>> differs from the host TSC rate. This patch fixes this problem by using >>>> the system time as elapsed_nsec. >>> For both this and patch 2, while at a first glance (and taking into >>> account just the visible patch context) what you say seems to >>> make sense, the explanation is far from sufficient namely when >>> looking at the function as a whole. For one, effects on existing >>> cases need to be explicitly described, in particular why SVM's TSC >>> ratio code works without that change (or whether it has been >>> broken all along, in which case these would become backporting >>> candidates; input from SVM maintainers would be appreciated >>> too). That may in particular mean being more specific about >>> what is actually wrong with scaling the host TSC here (i.e. in >>> which way both results differ), when supposedly that matches >>> what the hardware does when TSC ratio is supported. >> If elapsed_nsec is the time that guest has been running then how can >> get_s_time(), which is system time, be the right answer here? But what >> confuses me even more is that existing code is not doing that neither. >> >> Shouldn't elapsed_nsec be offset by d->arch.vtsc_offset on the get side? >> I.e. >> >> *elapsed_nsec = get_s_time() - d->arch.vtsc_offset? > Doesn't whether or not to adjust be the offset depend on d-arch.vtsc? We only use elapsed_nsec when vtsc is set, I think. In native case (vtsc=0) elapsed_nsec and d->arch.vtsc_offset are ignored. -boris