Re: [Xen-devel] [PATCH RESEND v1 0/7] Intel Processor Trace virtulization enabling

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

> > Here is a patch-series which adding Processor Trace enabling in XEN guest. 
> > You can get It's software developer manuals from:
> > https://software.intel.com/sites/default/files/managed/c5/15/architect
> > ure-instruction-set-extensions-programming-reference.pdf
> > In Chapter 5 INTEL PROCESSOR TRACE: VMX IMPROVEMENTS.
> >
> > Introduction:
> > Intel Processor Trace (Intel PT) is an extension of Intel Architecture that 
> > captures information about software execution using
> dedicated hardware facilities that cause only minimal performance 
> perturbation to the software being traced. Details on the Intel PT
> infrastructure and trace capabilities can be found in the Intel 64 and IA-32 
> Architectures Software Developer’s Manual, Volume 3C.
> >
> > The suite of architecture changes serve to simplify the process of 
> > virtualizing Intel PT for use by a guest software. There are two
> primary elements to this new architecture support for VMX support 
> improvements made for Intel PT.
> > 1. Addition of a new guest IA32_RTIT_CTL value field to the VMCS.
> >   — This serves to speed and simplify the process of disabling trace on VM 
> > exit, and restoring it on VM entry.
> > 2. Enabling use of EPT to redirect PT output.
> >   — This enables the VMM to elect to virtualize the PT output buffer using 
> > EPT. In this mode, the CPU will treat PT output
> addresses as Guest Physical Addresses (GPAs) and translate them using EPT. 
> This means that Intel PT output reads (of the ToPA
> table) and writes (of trace output) can cause EPT violations, and other 
> output events.
> >
> 
> A high level question, SDM vol 3 "Emulation of Intel PT Traced State"
> says:
> 
> "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."
> 
> The immediately follows that paragraph is an example of CR3 causing vmexit 
> which leads to missing packet. IIRC Xen does that,
> however the code as is doesn't seem to handle that at all.

Hi Wei,
    Intel PT can be exposed to guest only when EPT is enabled. In that case, 
CPU_BASED_CR3_LOAD_EXITING and CPU_BASED_CR3_STORE_EXITING would be clear, so 
"MOV CR3 " will not cause a vm-exit. It looks like don't need emulate the 
missing PIP by writing it into the guest output buffer.

Thanks,
Luwei Kang

> 
> Another thing is Xen's vmevent allows intercepting several other traced 
> states. It seems that a more generic framework is needed to
> make PT work with vmevent subsystem? What is your thought on that?
> 
> Wei.
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