11:33 p.m.
> > > > > 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