From ccda4494b0ea4b81b6b0c3e539a0bcf7e673c68c Mon Sep 17 00:00:00 2001

From: Eduardo Habkost <ehabkost@redhat.com>

Date: Thu, 5 Dec 2019 21:56:50 +0000

Subject: [PATCH 01/18] i386: Resolve CPU models to v1 by default

RH-Author: Eduardo Habkost <ehabkost@redhat.com>

Message-id: <20191205225650.772600-2-ehabkost@redhat.com>

Patchwork-id: 92907

O-Subject: [RHEL-AV-8.1.1 qemu-kvm PATCH 1/1] i386: Resolve CPU models to v1 by default

Bugzilla: 1787291 1779078 1779078

RH-Acked-by: Danilo de Paula <ddepaula@redhat.com>

RH-Acked-by: Igor Mammedov <imammedo@redhat.com>

RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1779078

Brew: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=25187823

Upstream: submitted, Message-Id: <20191205223339.764534-1-ehabkost@redhat.com>

When using `query-cpu-definitions` using `-machine none`,

QEMU is resolving all CPU models to their latest versions.  The

actual CPU model version being used by another machine type (e.g.

`pc-q35-4.0`) might be different.

In theory, this was OK because the correct CPU model

version is returned when using the correct `-machine` argument.

Except that in practice, this breaks libvirt expectations:

libvirt always use `-machine none` when checking if a CPU model

is runnable, because runnability is not expected to be affected

when the machine type is changed.

For example, when running on a Haswell host without TSX,

Haswell-v4 is runnable, but Haswell-v1 is not.  On those hosts,

`query-cpu-definitions` says Haswell is runnable if using

`-machine none`, but Haswell is actually not runnable using any

of the `pc-*` machine types (because they resolve Haswell to

Haswell-v1).  In other words, we're breaking the "runnability

guarantee" we promised to not break for a few releases (see

qemu-deprecated.texi).

To address this issue, change the default CPU model version to v1

on all machine types, so we make `query-cpu-definitions` output

when using `-machine none` match the results when using `pc-*`.

This will change in the future (the plan is to always return the

latest CPU model version if using `-machine none`), but only

after giving libvirt the opportunity to adapt.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1779078

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>

---

 qemu-deprecated.texi | 7 +++++++

 target/i386/cpu.c    | 8 +++++++-

 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi

index 4b4b742..534ebe9 100644

--- a/qemu-deprecated.texi

+++ b/qemu-deprecated.texi

@@ -374,6 +374,13 @@ guarantees must resolve the CPU model aliases using te

 ``alias-of'' field returned by the ``query-cpu-definitions'' QMP

 command.

+While those guarantees are kept, the return value of

+``query-cpu-definitions'' will have existing CPU model aliases

+point to a version that doesn't break runnability guarantees

+(specifically, version 1 of those CPU models).  In future QEMU

+versions, aliases will point to newer CPU model versions

+depending on the machine type, so management software must

+resolve CPU model aliases before starting a virtual machine.

 @node Recently removed features

 @appendix Recently removed features

diff --git a/target/i386/cpu.c b/target/i386/cpu.c

index 6dce6f2..863192c 100644

--- a/target/i386/cpu.c

+++ b/target/i386/cpu.c

@@ -3926,7 +3926,13 @@ static PropValue tcg_default_props[] = {

 };

-X86CPUVersion default_cpu_version = CPU_VERSION_LATEST;

+/*

+ * We resolve CPU model aliases using -v1 when using "-machine

+ * none", but this is just for compatibility while libvirt isn't

+ * adapted to resolve CPU model versions before creating VMs.

+ * See "Runnability guarantee of CPU models" at * qemu-deprecated.texi.

+ */

+X86CPUVersion default_cpu_version = 1;

 void x86_cpu_set_default_version(X86CPUVersion version)

 {

-- 

1.8.3.1