Re: [Xen-devel] Future x86 emulator direction

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 12/13/2016 07:10 PM, Andrew Cooper wrote:
> On 13/12/16 15:58, Razvan Cojocaru wrote:
>> Hello, and first of all thanks for the discussion!
>>
>>> Think of it a bit more like introducing a new action emulator (name
>>> definitely subject to improvement), which implements things such as
>>> wrmsr, cpuid, pagewalk, task_switch, etc.
>>>
>>> The vmexit helpers, given decode assistance from hardware, can directly
>>> call action->task_switch().  If insufficient information is available
>>> (e.g. LMSW on AMD), the helpers invoke the instruction emulator to work
>>> out what to do, and the instruction emulator would invoke the action
>>> emulator as part of its execute phase.
>>>
>>> Wherever possible, the action emulator should be guest-neutral, and
>>> ideally the single point of implementation of non-architectural actions
>>> such as "the vm_event subsystem is interested in this."
>>>
>>>> And to be honest, on the road towards
>>>> completion of the emulator I think the SVM/VMX insns are pretty
>>>> close to the end of the priority list.
>>> I'd expect them to show up frequently during introspection, although
>>> maybe I am wrong.  Razvan: Any thoughts?
>> I definitely think this is a good idea as far as introspection goes -
>> having a single contact surface with the underlying logic would be a
>> significant improvement.
>>
>> As for SVM/VMX instructions, we're interested in anything that is able
>> to trigger an EPT fault (and hence a mem_access event) - we've had our
>> share of adventures with VMX-specific instructions, so they're not low
>> priority for us.
> 
> In reality, this is any instruction if you set EPT NX on a page, I presume?
> 
> Do you have stats on which instructions you most frequently have to
> singlestep because of lack of emulator support, or is the spread
> essentially random?

Here's what I've gathered just now with nothing more than just starting
up, firing up Internet Explorer and Firefox, and shutting down a few
Windows 7 (32 and 64 bit) guests:

64-bit guest 1:

(XEN) Mem event emulation failed: d1v0 32bit @ 0023:7489f1f8 -> 66 0f 12
44 24 04 ba 00 00 00 00 66 0f 28 e8 66

64-bit guest 2:

(XEN) Mem event emulation failed: d2v1 32bit @ 0023:734cf031 -> 66 0f ef
c0 51 53 8b c1 83 e0 0f 85 c0 75 7f 8b

32-bit guest:

(XEN) Mem event emulation failed: d3v0 32bit @ 001b:7359f1f8 -> 66 0f 12
44 24 04 ba 00 00 00 00 66 0f 28 e8 66
(XEN) Mem event emulation failed: d3v0 32bit @ 001b:7359e7d5 -> 66 0f 12
0d 90 f1 59 73 f2 0f 59 c8 66 0f 12 15
(XEN) Mem event emulation failed: d3v0 32bit @ 0008:821d385f -> 0f 6e 06
0f 72 d0 18 0f ef 05 08 f6 32 82 0f 61

This is with 4.9-unstable master, and only took about 5 minutes per guest.


Thanks,
Razvan

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