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As explained in QEMU commit 4c257911dcc7c4189768e9651755c849ce9db4e8
intel-pt features should never be included in the CPU models as it was
not supported by KVM back then and even once it started to be supported,
users have to enable it by passing pt_mode=1 parameter to kvm_intel
module. The Icelake-* CPU models with intel-pt included were added to
QEMU 3.1.0 and removed right in the following 4.0.0 release (and even in
3.1.1 maintenance release).
In libvirt 6.10.0 I introduced 'removed' attribute for features included
in our CPU model definitions which we can use to drop intel-pt from
Icelake-* CPU models. Back then I explained we can safely do so only for
features which could never be enabled, which is not the case of intel-pt.
Theoretically, it could be possible to create an environment in which
QEMU would enable intel-pt without asking for it explicitly: it would
need to use a new enough kernel (not available at the time of QEMU
3.1.0) and pt_mode KVM parameter in combination with QEMU 3.1.0 running
a domain with q35 machine type and all that on a CPU which didn't really
exist at that time.
Migrating such domain to a host with newer SW stack including libvirt
with this patch applied would result in incompatible guest ABI (the
virtual CPU would lose intel-pt). However, QEMU changed its CPU models
unconditionally and thus migration would not work even without this
patch. That said, it is safe to follow QEMU and remove the feature from
Icelake-* CPU models in our cpu_map.
https://bugzilla.redhat.com/show_bug.cgi?id=1853972
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Tim Wiederhake <twiederh@redhat.com>
The pconfig feature was enabled in QEMU by accident in 3.1.0. All other
newer versions do not support it and it was removed from the
Icelake-Server CPU model in QEMU.
We don't normally change our CPU models even when QEMU does so to avoid
breaking migrations between different versions of libvirt. But we can
safely do so in this specific case. QEMU never supported enabling
pconfig so any domain which was able to start has pconfig disabled.
With a small compatibility hack which explicitly disables pconfig when
CPU model equals Icelake-Server in migratable domain definition, only
one migration scenario stays broken (and there's nothing we can do about
it): from any host to a host with libvirt < 5.10.0 and QEMU > 3.1.0.
https://bugzilla.redhat.com/show_bug.cgi?id=1749672
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
