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

["Dr. David Alan Gilbert" <dgilbert@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