From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1MndAs-00070F-M6 for qemu-devel@nongnu.org; Tue, 15 Sep 2009 14:54:46 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1MndAn-0006xc-Kj for qemu-devel@nongnu.org; Tue, 15 Sep 2009 14:54:45 -0400 Received: from [199.232.76.173] (port=50704 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1MndAn-0006xV-CM for qemu-devel@nongnu.org; Tue, 15 Sep 2009 14:54:41 -0400 Received: from fmmailgate01.web.de ([217.72.192.221]:33249) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1MndAm-00065g-OD for qemu-devel@nongnu.org; Tue, 15 Sep 2009 14:54:41 -0400 Message-ID: <4AAFE22F.6010606@web.de> Date: Tue, 15 Sep 2009 20:51:27 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <20090915113603.8313.49563.stgit@mchn012c.ww002.siemens.net> <20090915175041.GA2786@shareable.org> In-Reply-To: <20090915175041.GA2786@shareable.org> Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enig0E09BA5749D7414516CF1758" Sender: jan.kiszka@web.de Subject: [Qemu-devel] Re: [PATCH v3 0/5] Improve PC RTC emulation List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jamie Lokier Cc: Blue Swirl , Anthony Liguori , Dor Laor , Glauber Costa , qemu-devel@nongnu.org This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enig0E09BA5749D7414516CF1758 Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: quoted-printable Jamie Lokier wrote: > Jan Kiszka wrote: >> The aim of this series is to allow using the emulated PC RTC (MC146818= ) >> as a reliable time source for guests. This is particularly useful if t= he >> host runs NTP or has otherwise access to an accurate clock while the >> guest has not (no network, impossible to add an NTP implementation >> etc.). >=20 > I approve of these patches. >=20 > But still, how exactly is it useful, without NTP? Afaik, Linux does > not read the RTC except once at boot time, so it probably still needs > NTP or something like it to have reliable time. However, it does > _write_ the RTC every 11 minutes if running NTP - does that affect the > emulated behaviour? Or does the emulated RTC ignore writes? Nope, it accepts writes and simply gain a larger offset to the host clock this way. The same happens via -rtc base=3Dlocaltime or base=3D BTW. To make use of this new "accurate" RTC, Linux users should already be fine with adjtimex --adjust. We are currently evaluating this approach. The brute-force solution is still to run hwclock --hctosys in a loop. >=20 > Do any other OSes read the RTC more than once after boot? (At least our proprietary one does. :) ) Some Windows versions are known to use the RTC's periodic tick instead of the PIC or APIC as time source. They will also benefit from the potential better accuracy of the host-based RTC (which also affects its IRQ events). Jan --------------enig0E09BA5749D7414516CF1758 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.9 (GNU/Linux) Comment: Using GnuPG with SUSE - http://enigmail.mozdev.org iEYEARECAAYFAkqv4jIACgkQniDOoMHTA+kZhACfUyeHsa5pAWNN3YHu8f3f0gLl 1xwAnjZyT/sw7YoTcGt8uIBuzuPPZfOK =ETuK -----END PGP SIGNATURE----- --------------enig0E09BA5749D7414516CF1758--