On Thu, Jan 12, 2017 at 12:38:00PM +0000, Dr. David Alan Gilbert wrote: > * Eduardo Habkost (ehabkost@xxxxxxxxxx) wrote: > > On Mon, Jan 09, 2017 at 11:35:54AM +0000, Dr. David Alan Gilbert wrote: > > > * Eduardo Habkost (ehabkost@xxxxxxxxxx) 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 assume you are talking about the stepping change, when migrating from an old QEMU to a host that is _not_ on the blacklist. In this case, the guest won't see any changes: CPUID family/model/stepping will be kept the same for the whole life of the VM (even if it is shut down), thanks to the machine-type compatibility code. > I'm hoping the guest CPU ID is preserved with the TSX disabled until > a reboot? CPUID changes are effective immediately on migration. But guests often notice the change only on the next reboot. We could do something to make these problems less likely: including CPUID data on the migration stream. I have considered it on the past, but never implemented it. Maybe I should reconsider that. (This is another case where it would be interesting to have a mechanism to let the destination host abort migration early: we could make QEMU work in "enforce" mode when live-migrating, but using the migrated CPUID data. This way CPUID changes would never happen.) > > Dave > > > > > > > > > Any other combination? > > > > > > Dave > > > > > > > [1] https://sourceware.org/git/?p=glibc.git;a=commit;h=2702856bf45c82cf8e69f2064f5aa15c0ceb6359 > > > > > > > > --- > > > > Cc: dgilbert@xxxxxxxxxx > > > > Cc: fweimer@xxxxxxxxxx > > > > Cc: carlos@xxxxxxxxxx > > > > Cc: triegel@xxxxxxxxxx > > > > Cc: berrange@xxxxxxxxxx > > > > Cc: jdenemar@xxxxxxxxxx > > > > Cc: pbonzini@xxxxxxxxxx > > > > > > > > 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@xxxxxxxxxx / Manchester, UK > > > > -- > > Eduardo > -- > Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK -- Eduardo -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
