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Re: [Xen-devel] VMI: singlestep event not received
- To: Mathieu Tarral <mathieu.tarral@xxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 25 Apr 2019 20:34:59 +0100
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- Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 25 Apr 2019 19:35:11 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 25/04/2019 00:43, Mathieu Tarral wrote:
> On Wednesday 24 April 2019 14:00, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> wrote:
>
>> On 23/04/2019 22:59, Mathieu Tarral wrote:
>>
>>>>> The funny thing is that it's always at the same instruction that it
>>>>> fails, the 106th singlestep,
>>>>> at 0x806d32dc:
>>>>> [0x7c90e514]> s 0x806d32dc
>>>>> [0x806d32dc]> pd 10
>>>>> 0x806d32dc 890d8000feff mov dword [0xfffe0080], ecx
>>>>> This is a read of the APIC TPR, which is very commonly an operation
>>>>> accelerated by hardware (because without it, virtualising windows XP is
>>>>> exceedingly slow).
>>>> What is your CPU, and how exactly are you trying to singlestep. Is it
>>>> with MTF, or using the trap flag inside the guest?
>>>> My CPU is an Intel i7-8750H.
>>>> I'm using the libvmi API, which is enabling the MTF flag to singlestep, if
>>>> i judge by the debug logs.
>> Ok - that is a CoffeeLake.
>>
>> I've had a quick play with a simple example (XTF test and xen-access)
>> and MTF does appear to work fine with both TPR shadowing and full APIC
>> virtualisation. I've tried on Haswell, KabyLake and CoffeeLake.
> Thank you for testing.
>
>> Can you describe the exact VMI configuration you've got enabled -
>> specifically which other intercepts are active?
> My BreakpointManager class is dealing with VMI events.
> It registers 3 events:
> - int3
> - debug event (for hardware breakpoints)
> - singlestep event (for breakpoint recoil on int3)
>
> https://github.com/Wenzel/pyvmidbg/blob/1a2e97c238a96c98c19cdbbdefa6047be246e504/vmidbg/breakpoint.py#L20
What about control register intercepts, or memory permission restrictions?
I expect things are going wrong when the TPR access is emulated by Xen,
rather than completed directly by hardware, but I can't explain how
you'd be getting into that situation without some other VMI settings
being active at the same time.
~Andrew
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