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 12:09:54 -0400 Message-ID: <5617E6D2.10608@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> <20151009140005.GA10042@hzzhang-OptiPlex-9020.sh.intel.com> <5617F52C02000078000A9CCE@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: <5617F52C02000078000A9CCE@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 , Haozhong Zhang Cc: 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 11:11 AM, Jan Beulich wrote: >>>> On 09.10.15 at 16:00, wrote: >> On Fri, Oct 09, 2015 at 09:41:36AM -0400, Boris Ostrovsky 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? >>> >> Yes, I should minus d->arch.vtsc_offset here. > In which case - afaict - the code becomes identical to that of the > TSC_MODE_ALWAYS_EMULATE case as well as the > TSC_MODE_DEFAULT w/ d->arch.vtsc true. Which seems quite > unlikely to be correct. *elapsed_nsec = *gtsc_khz = 0; ? Because we are effectively in TSC_MODE_NEVER. That can't be right... -boris