RE: [PATCH v1 1/3] i386: Remove the limitation of IP payloads for Intel PT

[Kang, Luwei]

> >>>> No, it's not possible.  KVM doesn't have a say on what the
> >>>> processor writes in the tracing packets.
> >>> Can KVM refuse to enable packet generation if CSbase is not zero and
> >>> CPUID.(EAX=14H,ECX=0)[bit 31] seen by guest is different from host?
> >>
> >> Yes, but the processor could change operating mode (and hence CSbase)
> >> while tracing is active.  This is very unlikely, since it would
> >> require nonzero CS-base and a 32-bit host, but in principle not
> >> impossible (could be a firmware call, for example).
> >>
> >> The only solution is for KVM to accept both, and for QEMU to refuse a
> >> setting that does not match the host.
> >>
> >
> > So I need to add a patch in KVM to disabled the Intel PT when the
> > CSbase is not zero and the guest LIP different from the host. And this
> > limitation in qemu (disabled the PT when LIP is enabled in the host)
> > can be remove. Is that right?
> 
> No, if a feature cannot be emulated, that means it cannot be enabled unless it
> matches the host.  That's generally not a problem since Intel PT is usually 
> used
> only with "-cpu host".
> 

The limitation of LIP in qemu will mask off the Intel PT in KVM guest even with 
"-cpu host". e.g. This bit will be set in SnowRidge HW and later.

How about "-cpu cpu_model, +intel-pt" use case? The current value of Intel PT 
CPUID is a constant. Can we make the ICX CPUID as basic inforation(LIP is 0) 
and using "+intel-pt-lip" to make Intel PT work on the CPU which LIP is 1 on 
the host? As you mentioned before, Intel PT cannot be enabled in guest unless 
it matches the host.

Thanks,
Luwei Kang