From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keir Fraser Subject: Re: Xen timing mode Date: Tue, 23 Feb 2010 17:18:08 +0000 Message-ID: References: <5c3550fe1002230912j1a5d0650oa1e355652c8e3062@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <5c3550fe1002230912j1a5d0650oa1e355652c8e3062@mail.gmail.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Priya Cc: "xen-devel@lists.xensource.com" List-Id: xen-devel@lists.xenproject.org What do you mean by domain - virtual time? On 23/02/2010 17:12, "Priya" wrote: > Thanks ! >=20 > In that case, how does a para-virtualized user domain find the domain - > virtual time ?=20 >=20 > --Priya >=20 > On Tue, Feb 23, 2010 at 12:08 PM, Keir Fraser > wrote: >> Independent_wallclock is specific to Linux PV guests, and is set within = the >> guest. Timer_mode is specific to HVM guests and is a domain build-time >> config option. >>=20 >> =A0-- Keir >>=20 >> On 23/02/2010 16:50, "PBhat" wrote: >>=20 >>>=20 >>> A question about timing modes in Xen came up after reading the document= ation >>> on >>> http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight= =3D(int >>> er=20 >>> >> inter>=20 >>> rupts)|(timer) >>> configuration file options =A0and I was wondering if you could help me wi= th >>> it. >>>=20 >>> =A0 =A0 * I know that Xen has a facility on the user domains to change the >>> timing from something called the ' http://openskill.info/infobox.php?ID= =3D1451 >>> independent wallclock ' to 'dependent clock' and vice versa. The way to= do >>> this is to toggle the parameter /proc/sys/xen/independent_wallclock bet= ween >>> 0 (dependent) and 1 (independent) >>>=20 >>>=20 >>> =A0 =A0 * Now, the documentation on the configuration options for xen virtu= al >>> machine under the heading of Timers contains no mention of the above >>> mentioned facility, but instead talks about something called a 'timer_m= ode' >>>=20 >>>=20 >>> =A0timer_mode: Timer mode (0=3Ddelay virtual time when ticks are missed; >>> 1=3Dvirtual time is always wallclock time >>> =A0 =A0 =A0 timer_mode (default=3D1; Value=3D'TIMER_MODE') >>>=20 >>> Now my question is whether the timer mode =A0=3D 1 is equivalent to indepen= dent_ >>> wallclock? >>>=20 >>> I think the answer is no. While timer_mode =3D 1 decides whether the virt= ual >>> machine is able to read the Domain-0 time, the independent_wallclock de= cides >>> whether you need NTP sychronization on both (user and control) domains = or >>> not. >>>=20 >>> Namely, in the timer_mode =3D 1, the gettimeofday() called from the virtu= al >>> machine will return the domain virtual time, but time_mode =3D 0 >>> gettimeofday() instruction from the virtual machine will return the sys= tem >>> time. >>>=20 >>> The independent and dependent modes have an effect on whether the virtu= al >>> machines can use the NTP or any other synchronization of Domain-0. >>>=20 >>> Is my understanding correct? >>>=20 >>> I further want to know whether the working remains similar on Xen HVMs = also. >>> In the sense that are Hardware based virtual machines capable of readin= g the >>> Domain-0 system time in an analogous way? >>>=20 >>>=20 >>> Your help would be much appreciated. I am new to this area and I apolog= ize >>> if my questions seem too basic. >>=20 >>=20 >>=20 >=20 >=20