From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:45009) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z7GOR-0007jq-CL for qemu-devel@nongnu.org; Tue, 23 Jun 2015 01:05:08 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Z7GON-0006tN-56 for qemu-devel@nongnu.org; Tue, 23 Jun 2015 01:05:07 -0400 Received: from mout.web.de ([212.227.15.14]:63480) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z7GOM-0006qX-SJ for qemu-devel@nongnu.org; Tue, 23 Jun 2015 01:05:03 -0400 Message-ID: <5588E8F4.8080701@web.de> Date: Tue, 23 Jun 2015 07:04:52 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <55740B9C.6030107@web.de> <20150618202159.GK3874@thinpad.lan.raisama.net> <5586F67E.5010708@web.de> In-Reply-To: Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="mdUovPQ1OQLXidLqMLD8WM9sotisWosPt" Subject: Re: [Qemu-devel] [PATCH v5] i386: Introduce ARAT CPU feature List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Wanpeng Li , Eduardo Habkost Cc: Paolo Bonzini , Marcelo Tosatti , qemu-devel , kvm , "Michael S. Tsirkin" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --mdUovPQ1OQLXidLqMLD8WM9sotisWosPt Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: quoted-printable On 2015-06-23 04:50, Wanpeng Li wrote: >=20 >=20 > On 6/22/15 1:38 AM, Jan Kiszka wrote: >> On 2015-06-18 22:21, Eduardo Habkost wrote: >>> On Sun, Jun 07, 2015 at 11:15:08AM +0200, Jan Kiszka wrote: >>>> From: Jan Kiszka >>>> >>>> ARAT signals that the APIC timer does not stop in power saving state= s. >>>> As our APICs are emulated, it's fine to expose this feature to guest= s, >>>> at least when asking for KVM host features or with CPU types that >>>> include the flag. The exact model number that introduced the feature= is >>>> not known, but reports can be found that it's at least available sin= ce >>>> Sandy Bridge. >>>> >>>> Signed-off-by: Jan Kiszka >>> The code looks good now, but: what are the real consequences of >>> enabling/disabling the flag? What exactly guests use it for? >>> >>> Isn't this going to make guests have additional expectations about th= e >>> APIC timer that may be broken when live-migrating or pausing the VM? >> ARAT only refers to stopping of the timer in certain power states (whi= ch >> we do not even emulate IIRC). In that case, the OS is under risk of >> sleeping forever, thus need to look for a different wakeup source. >=20 > HPET will always be the default broadcast event device I think. But it's unused (under Linux) if per-cpu clockevents are unaffected by CLOCK_EVT_FEAT_C3STOP (x86-only "none-feature"), i.e. have ARAT set. And other guests may have other strategies to deal with missing ARAT. Again, the scenario for me was not a regular setup but some Jailhouse boot of Linux where neither a HPET nor a PIT are available as broadcast sources and Linux therefore refuses to switch to hires mode - in contrast to running on real hardware. Jan --mdUovPQ1OQLXidLqMLD8WM9sotisWosPt 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 iEYEARECAAYFAlWI6PQACgkQitSsb3rl5xQVgwCfUjIGF0cBuZs1h+o5Cidm1Iw6 eScAnA+VCxHx+HqICqEAlPIRFqbdadHy =NEz+ -----END PGP SIGNATURE----- --mdUovPQ1OQLXidLqMLD8WM9sotisWosPt--