RE: [PATCH v1 0/7] Implement support for external IPT monitoring

["Kang, Luwei" <luwei.kang@intel.com>]

> > > -----Original Message-----
> > > From: Tian, Kevin <kevin.tian@intel.com>
> > > Sent: Wednesday, June 17, 2020 9:35 AM
> > > To: Michał Leszczyński <michal.leszczynski@cert.pl>; Andrew Cooper
> > > <andrew.cooper3@citrix.com>
> > > Cc: Xen-devel <xen-devel@lists.xenproject.org>; Jan Beulich
> > > <jbeulich@suse.com>; Wei Liu <wl@xen.org>; Roger Pau Monné
> > > <roger.pau@citrix.com>; Nakajima, Jun <jun.nakajima@intel.com>;
> > > George Dunlap <george.dunlap@citrix.com>; Ian Jackson
> > > <ian.jackson@eu.citrix.com>; Julien Grall <julien@xen.org>; Stefano
> > > Stabellini <sstabellini@kernel.org>; Kang, Luwei
> > > <luwei.kang@intel.com>
> > > Subject: RE: [PATCH v1 0/7] Implement support for external IPT
> > > monitoring
> > >
> > > +Luwei, who developed PT for KVM and is the best one who can help
> > > review VMX changes from Intel side. Please include him in future
> > > post or discussion.
> > >
> > > > -----Original Message-----
> > > > From: Michał Leszczyński <michal.leszczynski@cert.pl>
> > > > Sent: Wednesday, June 17, 2020 2:48 AM
> > > > To: Andrew Cooper <andrew.cooper3@citrix.com>
> > > > Cc: Xen-devel <xen-devel@lists.xenproject.org>; Jan Beulich
> > > > <jbeulich@suse.com>; Wei Liu <wl@xen.org>; Roger Pau Monné
> > > > <roger.pau@citrix.com>; Nakajima, Jun <jun.nakajima@intel.com>;
> > > > Tian, Kevin <kevin.tian@intel.com>; George Dunlap
> > > > <george.dunlap@citrix.com>; Ian Jackson
> > > > <ian.jackson@eu.citrix.com>; Julien Grall <julien@xen.org>;
> > > > Stefano Stabellini <sstabellini@kernel.org>
> > > > Subject: Re: [PATCH v1 0/7] Implement support for external IPT
> > > > monitoring
> > > >
> > > > ----- 16 cze 2020 o 20:17, Andrew Cooper andrew.cooper3@citrix.com
> > > > napisał(a):
> > > >
> > > > > On 16/06/2020 16:16, Michał Leszczyński wrote:
> > > > > When this subject was broached on xen-devel before, one issue
> > > > > was the fact that all actions which are intercepted don't end up
> > > > > writing any appropriate packets.  This is perhaps less of an
> > > > > issue for this example, where the external agent can see VMExits
> > > > > in the trace, but it still results in missing information.  (It
> > > > > is a major problem for PT within the guest, and needs Xen's
> > > > > intercept/emulation framework being updated to be PT-aware so it
> > > > > can fill in the same packets which hardware would have done for
> > > > > equivalent actions.)
> > > >
> > > > Ok, this sounds like a hard issue. Could you point out what could
> > > > be the particular problematic cases? For instance, if something
> > > > would alter EIP/RIP or CR3 then I belive it would still be
> > > > recorded in PT trace (i.e. these values will be logged on VM entry).
> >
> > e.g. If a VM exit is taken on a guest write to CR3 (including “MOV
> > CR3” as well as task switches), the PIP packet normally generated on the CR3
> write will be missing. The PIP packet needs to be written to the PT buffer by
> software. Another example is VM-exit taken on RDTSC.
> >
> > For VM introspection, all the Intel PT packets may need to emulated by
> software. Some description in SDM as below:
> > If a VMM emulates an element of processor state by taking a VM exit on
> reads and/or writes to that piece of state, and the state element impacts Intel
> PT packet generation or values, it may be incumbent upon the VMM to insert
> or modify the output trace data.
> 
> I got the impression that IPT was mostly useful together with introspection, as
> you can then get events from trapped instructions (and likely emulated) from
> the introspection interface, while being able to get the processor trace for non-
> trapped events.
> 
> I'm not sure whether there would be corner cases with trapped instructions
> not being handled by the introspection framework.
> 
> How does KVM deal with this, do they insert/modify trace packets on trapped
> and emulated instructions by the VMM?

The KVM includes instruction decoder and emulator(arch/x86/kvm/emulate.c), and the guest's memory can be set to write-protect as well. But it doesn't support Intel PT packets software emulator. For KVM, the Intel PT feature will be exposed to KVM guest and KVM guest can use Intel PT feature like native.

Thanks,
Luwei Kang