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

[Eduardo Habkost]

On Mon, Sep 28, 2020 at 12:42:37PM +0000, Kang, Luwei wrote:
> > >>>> 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.

This behavior can and should be changed.

> 
> 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.

This makes sense, but you can also make each CPU model set a
default for the LIP bit.  "-cpu SnowRidge,+intel-pt" could set
LIP=1 by default.

-- 
Eduardo