From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47955) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cRedv-0006n8-9J for qemu-devel@nongnu.org; Thu, 12 Jan 2017 07:38:19 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cRedq-0002aM-Cl for qemu-devel@nongnu.org; Thu, 12 Jan 2017 07:38:11 -0500 Received: from mx1.redhat.com ([209.132.183.28]:52372) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cRedq-0002Yn-4A for qemu-devel@nongnu.org; Thu, 12 Jan 2017 07:38:06 -0500 Received: from int-mx11.intmail.prod.int.phx2.redhat.com (int-mx11.intmail.prod.int.phx2.redhat.com [10.5.11.24]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 4605081240 for ; Thu, 12 Jan 2017 12:38:06 +0000 (UTC) Date: Thu, 12 Jan 2017 12:38:00 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20170112123800.GC2513@work-vm> References: <20170108194041.10908-1-ehabkost@redhat.com> <20170109113553.GC2541@work-vm> <20170112123308.GH3565@thinpad.lan.raisama.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170112123308.GH3565@thinpad.lan.raisama.net> Subject: Re: [Qemu-devel] [PATCH 0/5] Use non-blacklisted family/model/stepping for Haswell CPU model List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eduardo Habkost Cc: qemu-devel@nongnu.org, libvir-list@redhat.com, fweimer@redhat.com, carlos@redhat.com, triegel@redhat.com, berrange@redhat.com, jdenemar@redhat.com, pbonzini@redhat.com * Eduardo Habkost (ehabkost@redhat.com) wrote: > On Mon, Jan 09, 2017 at 11:35:54AM +0000, Dr. David Alan Gilbert wrote: > > * Eduardo Habkost (ehabkost@redhat.com) wrote: > > > A recent glibc commit[1] added a blacklist to ensure it won't use > > > TSX on hosts that are known to have a broken TSX implementation. > > > > > > Our existing Haswell CPU model has a blacklisted > > > family/model/stepping combination, so it has to be updated to > > > make sure guests will really use TSX. This is done by patch 5/5. > > > > > > However, to do this safely we need to ensure the host CPU is not > > > a blacklisted one, so we won't mislead guests by exposing > > > known-to-be-good FMS values on a known-to-be-broken host. This is > > > done by patch 3/5. > > > > I'd just like to mke sure I understand the way this will fail in a migration; > > lets say we have a guest that doesn't have the new libc and hosts > > with a blacklisted CPU, and -cpu Haswell. > > > > If I understand correctly then: > > a) With 'enforce' the destination qemu will fail to start > > printing an error about the host lack of tsx feature. > > Yes. > > > b) Without 'enforce' the destination will start but print > > the same error as a warning, but the guest will probably > > break as soon as it tries to use a tsx feature? > > Yes. The general rule is: without "enforce", live migration can > break in unpredictable ways. > > Without "enforce", QEMU will print a warning, and the VCPU will > run _without_ the TSX features on CPUID. If we're live-migrating, > it may break the guest if it tries to use a TSX feature, or break > migration if a TSX-related bit is already set on a MSR. OK, but you've been telling people to use "enforce" long enough that they should have listened. Are there any other cases we have to worry about; lets say a VM with the new libc being migrated from an older QEMU, it suddenly changes CPU ID to one that's supported; what happens? I'm hoping the guest CPU ID is preserved with the TSX disabled until a reboot? Dave > > > > > Any other combination? > > > > Dave > > > > > [1] https://sourceware.org/git/?p=glibc.git;a=commit;h=2702856bf45c82cf8e69f2064f5aa15c0ceb6359 > > > > > > --- > > > Cc: dgilbert@redhat.com > > > Cc: fweimer@redhat.com > > > Cc: carlos@redhat.com > > > Cc: triegel@redhat.com > > > Cc: berrange@redhat.com > > > Cc: jdenemar@redhat.com > > > Cc: pbonzini@redhat.com > > > > > > Eduardo Habkost (5): > > > i386: Add explicit array size to x86_cpu_vendor_words2str() > > > i386: host_vendor_fms() helper function > > > i386/kvm: Blacklist TSX on known broken hosts > > > pc: Add 2.9 machine-types > > > i386: Change stepping of Haswell to non-blacklisted value > > > > > > include/hw/i386/pc.h | 6 ++++++ > > > target/i386/cpu.h | 1 + > > > hw/i386/pc_piix.c | 15 ++++++++++++--- > > > hw/i386/pc_q35.c | 13 +++++++++++-- > > > target/i386/cpu.c | 32 ++++++++++++++++++++++---------- > > > target/i386/kvm.c | 17 +++++++++++++++++ > > > 6 files changed, 69 insertions(+), 15 deletions(-) > > > > > > -- > > > 2.11.0.259.g40922b1 > > > > > -- > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > > -- > Eduardo -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK