On Tue, Jan 16, 2018 at 06:10:17AM +0000, Kang, Luwei wrote:
On Mon, Jan 15, 2018 at 12:04:55 -0200, Eduardo Habkost wrote:
CCing libvirt developers. ... This case is slightly more problematic, however: the new feature is actually migratable (under very controlled circumstances) because of patch 2/2, but it is not migration-safe[1]. This means libvirt shouldn't include it in "host-model" expansion (which uses the query-cpu-model-expansion QMP command) until we make the feature migration-safe.
For QEMU, this means the feature shouldn't be returned by "query-cpu-model-expansion type=static model=max" (but it can be returned by "query-cpu-model-expansion type=full model=max").
Jiri, it looks like libvirt uses type=full on query-cpu-model-expansion on x86. It needs to use type=static[2], or it will have no way to find out if a feature is migration-safe or not. ... [2] It looks like libvirt uses type=full because it wants to get all QOM property aliases returned. In this case, one solution for libvirt is to use:
static_expansion = query_cpu_model_expansion(type=static, model) all_props = query_cpu_model_expansion(type=full, static_expansion)
This is exactly what libvirt is doing (with model = "host") ever since query-cpu-model-expansion support was implemented for x86.
Oh, now I see that the x86 code uses QEMU_MONITOR_CPU_MODEL_EXPANSION_STATIC_FULL and not just QEMU_MONITOR_CPU_MODEL_EXPANSION_FULL. Nice!
So, I need to add Intel PT feature in "X86CPUDefinition builtin_x86_defs[]" so that we can get this CPUID in specific CPU model not only "-cpu host". Is that right?
The problem is that you won't be able to add intel-pt to any CPU model unless the feature is made migration-safe (by not calling kvm_arch_get_supported_cpuid() from cpu_x86_cpuid()). What's missing here is to either: (a) make cpu_x86_cpuid() return host-independent data (it can be constant, or it can be configurable on the command-line); or (b) add a mechanism to skip intel-pt from "query-cpu-model-expansion type=static". Probably (a) is easier to implement, and it also makes the feature more useful (by making it migration-safe).
Intel PT is first supported in Intel Core M and 5th generation Intel Core processors that are based on the Intel micro-architecture code name Broadwell but Intel PT use EPT is first supported in Ice Lake. Intel PT virtualization depend on PT use EPT. I will add Intel PT to "Broadwell" CPU model and later to make sure a "Broadwell" guest can use Intel PT if the host is Ice Lake.
The "if the host is Ice Lake" part is problematic. On migration-safe CPU models (all of them except "max" and "host"), the data seen on CPUID can't depend on the host at all. It should depend only on the machine-type + command-line options. -- Eduardo