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

[Eduardo Habkost]

On Tue, Sep 29, 2020 at 02:28:48AM +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.
> 
> I have a question on how to set LIP=1 in SnowRidge by default. 
> 1. Set LIP in "builtin_x86_defs[]" SnowRidge CPU model. The LIP included in 
> CPUID.(eax=14x,ecx=0)ecx[bit31] and a new leaf needs to be added.
> 2. Checking the CPU model in the later software flow and set the LIP bit if 
> the CPU model is Snowridge. And we also need to add more CPU model's checking 
> for new CPUs.
> 
> What is your opinion?
> 

1 is preferred.  Any CPU-model-specific data should be
represented as data inside builtin_x86_defs, not code.

-- 
Eduardo