From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joanna Rutkowska Subject: Re: A clocksource question Date: Fri, 12 Mar 2010 01:34:40 +0100 Message-ID: <4B998C20.9060803@invisiblethingslab.com> References: <4B962748.90609@invisiblethingslab.com> <4B96DB69.10101@goop.org> <4B981997.4030605@invisiblethingslab.com> <56cc3abf-21f6-4178-b00e-0331538080d9@default 4FA716B1526C7C4DB0375C6DADBC4EA3554D53F1B0@LONPMAILBOX01.citrite.net> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============1241914541==" Return-path: In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Dan Magenheimer Cc: Ian Pratt , xen-devel@lists.xensource.com, Jeremy Fitzhardinge List-Id: xen-devel@lists.xenproject.org This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --===============1241914541== Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enig8FED723F55A5E0ECFD382864" This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enig8FED723F55A5E0ECFD382864 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable On 03/11/2010 01:44 AM, Dan Magenheimer wrote: >> From: Ian Pratt [mailto:Ian.Pratt@eu.citrix.com] >> Sent: Wednesday, March 10, 2010 5:22 PM >> To: Dan Magenheimer; Joanna Rutkowska; Jeremy Fitzhardinge >> Cc: Ian Pratt; xen-devel@lists.xensource.com >> Subject: RE: [Xen-devel] A clocksource question >> >>> The TSC delta is troubling... if you hadn't said you had turned >>> off all power management, I would have guessed a problem with >>> C-state management. Maybe Xen is discovering some power management >>> capability not visible in BIOS settings? >> >> Xen uses the architectural C state mechanism, bypassing ACPI. >> Use "max_cstate=3D0" >=20 > Yes! The Core 2 Duo fooled me. There are some versions > of Core 2 Duo ("Conroe") that don't support C3-state and > some ("Merom") that do support C3-state. And the code > to "recover" from C3, which I think was added before 3.4 > was released, has been observed to be very poor in its > attempt to reset TSC after C3 to a reasonable value. I > didn't think it was THAT poor though! See: > http://lists.xensource.com/archives/html/xen-devel/2009-10/msg01414.htm= l=20 >=20 > (AFAIK, this is not fixed in 4.0, though the new default > rdtsc emulation may mask the problem by ensuring TSC always > moves forward, even across different processors.) >=20 > So that may explain the TSC delta. Since the pv clocksource > algorithm is dependent in part on the hardware TSC being > synced reasonably well by Xen, that may also explain > other clock strangeness. >=20 > P.S. Joanna -- max_cstate=3D0 must be specified on the > Xen boot line in grub.conf, not in the vm.cfg or grub.conf > of the guest. Dan, I tried passing this argument to Xen, of course and it didn't help. I still think it's a Dom0-related problem rather than a hypervisor, because when I was using xen/stable-2.6.32-based kernel for a moment, I'm pretty sure it worked fine and I didn't have any problem with the clocksource. On the same hypervisor, when I used xen/stable-2.6.31 kernel, the problem was present. (I had to switch back to 2.6.31 because, back then, the 2.6.32 didn't have pciback, which was important for me; and recently I decided to downgrade to Xen 3.4.2, for its presumed stability, and so 2.6.31 is the only option now). joanna. --------------enig8FED723F55A5E0ECFD382864 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Comment: Using GnuPG with Fedora - http://enigmail.mozdev.org/ iEYEARECAAYFAkuZjCAACgkQORdkotfEW85umgCfRs9JkzOaVpntO5WSNh5xgEYN CykAnRQ4NZvjE0ELIXAedg4Ht/eQ6k/d =M8o8 -----END PGP SIGNATURE----- --------------enig8FED723F55A5E0ECFD382864-- --===============1241914541== Content-Type: text/plain; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Disposition: inline _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel --===============1241914541==--