Re: [libvirt] [PATCH 0/5] Use non-blacklisted family/model/stepping for Haswell CPU model

On Mon, Jan 09, 2017 at 11:35:54AM +0000, Dr. David Alan Gilbert wrote: > * Eduardo Habkost (ehabkost(a)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.

> > Any other combination? > > Dave > > > [1] https://sourceware.org/git/?p=glibc.git;a=commit;h=2702856bf45c82cf8e69f2... > > > > --- > > Cc: dgilbert(a)redhat.com > > Cc: fweimer(a)redhat.com > > Cc: carlos(a)redhat.com > > Cc: triegel(a)redhat.com > > Cc: berrange(a)redhat.com > > Cc: jdenemar(a)redhat.com > > Cc: pbonzini(a)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(a)redhat.com / Manchester, UK -- Eduardo