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@xxxxxxxxxx> --- I don't think this should block QEMU 4.2.0 from being tagged. The bug is present since 4.1.0, so we can fix it in 4.1.2 and/or 4.2.1. --- qemu-deprecated.texi | 8 ++++++++ target/i386/cpu.c | 8 +++++++- 2 files changed, 15 insertions(+), 1 deletion(-) diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi index 4b4b7425ac..b42d8b3c5f 100644 --- a/qemu-deprecated.texi +++ b/qemu-deprecated.texi @@ -374,6 +374,14 @@ 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 69f518a21a..54e7f18a09 100644 --- a/target/i386/cpu.c +++ b/target/i386/cpu.c @@ -3924,7 +3924,13 @@ 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) { -- 2.23.0 -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
