Unlike host-model, host-passthrough does not do any check, it just passes "-cpu host" blindly. Therefore, lack of flags can and will result in crashes---SIGSEGV if it happens in userspace, but worse if for example the destination doesn't have an MSR that should be there according to the kernel.
Paolo Il mar 6 ago 2019, 18:22 Dr. David Alan Gilbert <dgilb...@redhat.com> ha scritto: > * Paolo Bonzini (pbonz...@redhat.com) wrote: > > host-passthrough documentation menions that the source and destination > > hosts are not identical in both hardware and configuration. > Configuration > > actually includes microcode version and QEMU version, but this is not > > clear so make it explicit > > > > Signed-off-by: Paolo Bonzini <pbonz...@redhat.com> > > --- > > docs/formatdomain.html.in | 12 ++++++------ > > 1 file changed, 6 insertions(+), 6 deletions(-) > > > > diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in > > index f5c882141a..6d084d7c04 100644 > > --- a/docs/formatdomain.html.in > > +++ b/docs/formatdomain.html.in > > @@ -1590,8 +1590,8 @@ > > hand, the ABI provided to the guest is reproducible. During > > migration, complete CPU model definition is transferred to the > > destination host so the migrated guest will see exactly the > same CPU > > - model even if the destination host contains more capable CPUs > for > > - the running instance of the guest; but shutting down and > restarting > > + model for the running instance of the guest, even if the > destination > > + host contains more capable CPUs or newer kernel; but shutting > down and restarting > > the guest may present different hardware to the guest > according to > > the capabilities of the new host. Prior to libvirt 3.2.0 and > QEMU > > 2.9.0 detection of the host CPU model via QEMU is not > supported. > > @@ -1625,10 +1625,10 @@ > > environment cannot be reproduced on different hardware. Thus, > if you > > hit any bugs, you are on your own. Further details of that > CPU can > > be changed using <code>feature</code> elements. Migration of > a guest > > - using host-passthrough is dangerous if the source and > destination > > - hosts are not identical in both hardware and configuration. > If such > > - a migration is attempted then the guest may hang or crash upon > > - resuming execution on the destination host.</dd> > > + using host-passthrough is dangerous if the source and > destination hosts > > + are not identical in both hardware, QEMU version, microcode > version > > + and configuration. If such a migration is attempted then the > guest may > > + hang or crash upon resuming execution on the destination > host.</dd> > > If libvirt is driving qemu with 'exact' on the cpu option, under what > situation would it hang/crash on the destination? > All of the microcode changes I think have caused a flag to appear, and > thus it would be required on the destination. > > Dave > > > </dl> > > > > Both <code>host-model</code> and <code>host-passthrough</code> > modes > > -- > > 2.21.0 > > > -- > Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK >
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