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

[Eduardo Habkost]

On Thu, Jan 12, 2017 at 12:38:00PM +0000, Dr. David Alan Gilbert wrote:
> * Eduardo Habkost (ehabkost at redhat.com) wrote:
> > On Mon, Jan 09, 2017 at 11:35:54AM +0000, Dr. David Alan Gilbert wrote:
> > > * Eduardo Habkost (ehabkost at 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 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 at redhat.com
> > > > Cc: fweimer at redhat.com
> > > > Cc: carlos at redhat.com
> > > > Cc: triegel at redhat.com
> > > > Cc: berrange at redhat.com
> > > > Cc: jdenemar at redhat.com
> > > > Cc: pbonzini at 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 at redhat.com / Manchester, UK
> > 
> > -- 
> > Eduardo
> --
> Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK

-- 
Eduardo