From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: [PATCH] Don't call cpu_synchronize_state() in apic_init_reset() Date: Thu, 24 Sep 2009 10:15:15 +0200 Message-ID: <4ABB2A93.9040905@web.de> References: <1253631112-26124-1-git-send-email-gleb@redhat.com> <1253631112-26124-3-git-send-email-gleb@redhat.com> <4AB9E3BC.7060304@redhat.com> <4ABA399F.6030003@web.de> <4ABA3C14.7080803@redhat.com> <4ABA42A1.4010904@web.de> <4ABB2597.7090107@redhat.com> <20090924080312.GB9832@redhat.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enig54F19FAEB5E9D3E03C79ABF8" Cc: Avi Kivity , kvm@vger.kernel.org To: Gleb Natapov Return-path: Received: from fmmailgate02.web.de ([217.72.192.227]:56864 "EHLO fmmailgate02.web.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751890AbZIXIP0 (ORCPT ); Thu, 24 Sep 2009 04:15:26 -0400 In-Reply-To: <20090924080312.GB9832@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enig54F19FAEB5E9D3E03C79ABF8 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Gleb Natapov wrote: > On Thu, Sep 24, 2009 at 10:53:59AM +0300, Avi Kivity wrote: >> On 09/23/2009 06:45 PM, Jan Kiszka wrote: >>>> Functions calling each other in the same subsystem can rely on calle= rs >>>> calling cpu_synchronize_state(). Across subsystems, that's another >>>> matter, exported functions should try not to rely on implementation >>>> details of their callers. >>>> >>>> (You might argue that the apic is not separate subsystem wrt an x86 = cpu, >>>> and I'm not sure I have a counterargument) >>>> >>> I do accept this argument. It's just that my feeling is that we are >>> lacking proper review of the required call sites of cpu_sychronize_st= ate >>> and rather put it where some regression popped up (and that only in >>> qemu-kvm...). >> That's life... >> >>> The new rule is: Synchronize the states before accessing registers (o= r >>> in-kernel devices) the first time after a vmexit to user space. >> No, the rule is: synchronize state before accessing registers. >> Extra synchronization is cheap, while missing synchronization is >> very expensive. >> > So should we stick cpu_synchronize_state() before each register > accesses? I think it is reasonable to omit it if all callers do it > already. >=20 >>> But, >>> e.g., I do not see where we do this on CPU reset. >> That's a bug. >> > Only if kvm support cpus without apic. Otherwise CPU is reset by=20 > apic_reset() and cpu_synchronize_state() is called there. No, that's not enough if cpu_reset() first fiddles with some registers that may later on be overwritten on cpu_synchronize_state() with the old in-kernel state. At least in theory, haven't checked yet what happens in reality. That's why not synchronizing properly is "expensive" (or broken IOW). Jan --------------enig54F19FAEB5E9D3E03C79ABF8 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 iEYEARECAAYFAkq7KpgACgkQitSsb3rl5xRxkQCgnFW6agmCblrxVIDZNRVUGz6P OnEAoLTCvzMkQDKDJlkXdOFIredfH8Jt =p+aD -----END PGP SIGNATURE----- --------------enig54F19FAEB5E9D3E03C79ABF8--