[PULL,1/3] i386: Resolve CPU models to v1 by default
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Message ID 20191220192031.2226378-2-ehabkost@redhat.com
State New
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  • [PULL,1/3] i386: Resolve CPU models to v1 by default
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Commit Message

Eduardo Habkost Dec. 20, 2019, 7:20 p.m. UTC
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>
Message-Id: <20191205223339.764534-1-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 qemu-deprecated.texi | 8 ++++++++
 target/i386/cpu.c    | 8 +++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

Patch
diff mbox series

diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
index 62680f7bd5..7b94653cef 100644
--- a/qemu-deprecated.texi
+++ b/qemu-deprecated.texi
@@ -367,6 +367,14 @@  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 0a9ac65974..ba5e9d5d6b 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -3981,7 +3981,13 @@  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)
 {