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From ccda4494b0ea4b81b6b0c3e539a0bcf7e673c68c Mon Sep 17 00:00:00 2001
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From: Eduardo Habkost <ehabkost@redhat.com>
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Date: Thu, 5 Dec 2019 21:56:50 +0000
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Subject: [PATCH 01/18] i386: Resolve CPU models to v1 by default
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RH-Author: Eduardo Habkost <ehabkost@redhat.com>
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Message-id: <20191205225650.772600-2-ehabkost@redhat.com>
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Patchwork-id: 92907
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O-Subject: [RHEL-AV-8.1.1 qemu-kvm PATCH 1/1] i386: Resolve CPU models to v1 by default
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Bugzilla: 1787291 1779078 1779078
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RH-Acked-by: Danilo de Paula <ddepaula@redhat.com>
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RH-Acked-by: Igor Mammedov <imammedo@redhat.com>
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1779078
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Brew: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=25187823
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Upstream: submitted, Message-Id: <20191205223339.764534-1-ehabkost@redhat.com>
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When using `query-cpu-definitions` using `-machine none`,
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QEMU is resolving all CPU models to their latest versions. The
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actual CPU model version being used by another machine type (e.g.
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`pc-q35-4.0`) might be different.
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In theory, this was OK because the correct CPU model
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version is returned when using the correct `-machine` argument.
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Except that in practice, this breaks libvirt expectations:
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libvirt always use `-machine none` when checking if a CPU model
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is runnable, because runnability is not expected to be affected
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when the machine type is changed.
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For example, when running on a Haswell host without TSX,
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Haswell-v4 is runnable, but Haswell-v1 is not. On those hosts,
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`query-cpu-definitions` says Haswell is runnable if using
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`-machine none`, but Haswell is actually not runnable using any
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of the `pc-*` machine types (because they resolve Haswell to
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Haswell-v1). In other words, we're breaking the "runnability
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guarantee" we promised to not break for a few releases (see
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qemu-deprecated.texi).
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To address this issue, change the default CPU model version to v1
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on all machine types, so we make `query-cpu-definitions` output
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when using `-machine none` match the results when using `pc-*`.
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This will change in the future (the plan is to always return the
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latest CPU model version if using `-machine none`), but only
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after giving libvirt the opportunity to adapt.
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Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1779078
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Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
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Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
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---
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qemu-deprecated.texi | 7 +++++++
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target/i386/cpu.c | 8 +++++++-
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2 files changed, 14 insertions(+), 1 deletion(-)
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diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
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index 4b4b742..534ebe9 100644
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--- a/qemu-deprecated.texi
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+++ b/qemu-deprecated.texi
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@@ -374,6 +374,13 @@ guarantees must resolve the CPU model aliases using te
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``alias-of'' field returned by the ``query-cpu-definitions'' QMP
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command.
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+While those guarantees are kept, the return value of
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+``query-cpu-definitions'' will have existing CPU model aliases
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+point to a version that doesn't break runnability guarantees
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+(specifically, version 1 of those CPU models). In future QEMU
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+versions, aliases will point to newer CPU model versions
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+depending on the machine type, so management software must
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+resolve CPU model aliases before starting a virtual machine.
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@node Recently removed features
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@appendix Recently removed features
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diff --git a/target/i386/cpu.c b/target/i386/cpu.c
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index 6dce6f2..863192c 100644
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--- a/target/i386/cpu.c
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+++ b/target/i386/cpu.c
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@@ -3926,7 +3926,13 @@ static PropValue tcg_default_props[] = {
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};
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-X86CPUVersion default_cpu_version = CPU_VERSION_LATEST;
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+/*
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+ * We resolve CPU model aliases using -v1 when using "-machine
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+ * none", but this is just for compatibility while libvirt isn't
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+ * adapted to resolve CPU model versions before creating VMs.
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+ * See "Runnability guarantee of CPU models" at * qemu-deprecated.texi.
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+ */
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+X86CPUVersion default_cpu_version = 1;
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void x86_cpu_set_default_version(X86CPUVersion version)
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{
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--
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1.8.3.1
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