Re: [libvirt] [PATCH RESEND v1 1/2] i386: Add Intel Processor Trace feature support

> > > > > > 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()). > > > > Hi Eduardo, > > Have some question need you help clear. What is > > "migration-safe" feature mean? I find all the feature which > > included in CPU model (builtin_x86_defs[]) will make > > "xcc->migration_safe = true;" in > > x86_cpu_cpudef_class_init(). If create a Skylake guest on > > Skylake HW and live migrate this guest to another machine > > with old HW(e.g Haswell). Can we think the new feature or > > cpu model(Skylake guest) which only supported in Skylake HW > > is safe? > > I mean matching the requirements so we can say the feature is migration-safe, that means exposing the same CPUID data to the > guest on any host, if the machine-type + command-line is the same. The data on CPUID[7] is OK (it changes according to the > command-line options only), but the data exposed on CPUID[14h] on this patch is not migration-safe (it changes depending on the > host it is running). > Many thanks for your clarification. I think I have understood. But CPUID[14h] is depend on CPUID[7].EBX[bit25] and it would be zero if CPUID[7].EBX[bit25] is not set. Or what you concerned is make live migration on two different HW which all support Intel PT virtualization but have different CPUID[14h] result? This may need to think about.

Thanks, Luwei Kang > > > > > > > > > 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". > > > > ==> it can be constant: > > I think it shouldn't be constant because Intel PT virtualization can only supported in Ice Lake hardware now. Intel PT cpuid info > would be mask off on old platform. > > ==> or it can be configurable on the command-line: > > Because of I didn't add this feature in any CPU model. We only can enable this feature by "-cpu Skylake-Server,+intel-pt" at > present. > > Note that I'm talking about CPUID[14h], not CPUID[7]. The CPUID[7] bits are safe because they are set according to the CPU model > + command-line options only. The bits on CPUID[14h] change depending on the host and are not migration-safe. CPUID[7].EBX[bit > 25] is just the (already configurable) bit that enables the migration-unsafe code for CPUID[14h]. > > > > > What about add a new cpu model name "Icelake" and add PT in this. Is that would be migration safe? > > It won't, because of the CPUID[14h] code that makes it unsafe to migrate between hosts with different Intel-PT capabilities (i.e. > different data on CPUID[14h]). > > > > > > Thanks, > > Luwei Kang > > > > > 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 > > -- > Eduardo