Re: [Xen-devel] [PATCH 0/6] Intel Processor Trace virtulization enabling

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 21/10/17 21:02, Luwei Kang wrote:
> Hi All,
>
> 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/architecture-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.

Hello,

Having read the new proposed extensions, I've got some architecture
questions before diving into the patches themselves.

First of all, is this technology expected to end up in Icelake, or
something later?

In Vol 3, the existing VMX support describes a number of scenarios
(system wide, VMM-only, VM-only, guest aware), which require the use of
MSR load lists to atomically alter the IA32_RTIT_* msrs.

Obviously, system wide mode is incompatible with also allowing the guest
to use PT itself, but what about Xen wanting to use PT for itself, and
for the guest to use PT as well?

Previously, this appears to be possible using the MSR load lists (albeit
with Xen needing to shadow the ToPA records to cause the packet stream
to end up in the right place).

However, the new VM consistency checks require that using EPT
redirection requires clear/load CTL on exit/entry be set, and having
load on entry set requires the host TraceEn to be clear.

Therefore, as far as I can see, allowing a guest to use PT via EPT now
prohibits Xen also using PT for its own purposes outside of non-root mode.

Is this intentional and/or expected, or have I misunderstood something
in the manuals?

Thanks,

~Andrew

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