Re: [Xen-devel] VM_EVENT_FLAG_SET_REGISTERS and sync_vcpu_execstate()

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 08/08/2016 03:01 PM, Andrew Cooper wrote:
> On 08/08/16 11:31, Razvan Cojocaru wrote:
>> Hello,
>>
>> We've been mostly setting registers by using xc_vcpu_setcontext():
>>
>> https://github.com/razvan-cojocaru/libbdvmi/blob/master/src/bdvmixendriver.cpp#L504
>>
>> but lately trying to push as much of that as possible to the
>> VM_EVENT_FLAG_SET_REGISTERS-related code (i.e. via the vm_event replies)
>> to save processing time.
>>
>> So with the xc_vcpu_setcontext() call removed, I've found that there are
>> cases where vm_event_set_registers() won't work properly unless I kept
>> the xc_vcpu_getcontext() call. This would appear to be not because of
>> anything that arch_get_info_guest() does (please see the implementation
>> for XEN_DOMCTL_getvcpucontext), but because of the vcpu_pause() call, or
>> more specifically, because of its calling sync_vcpu_execstate().
>>
>> So it would appear that a sync_vcpu_execstate(v) call is necessary at
>> the start of vm_event_set_registers() for the vcpu struct instance to be
>> synchronized with the current VCPU state.
>>
>> Any objections to a patch with this simple modification?
> 
> It would be helpful to identify exactly what is currently going wrong,
> and why sync_vcpu_execstate(v) helps.

I've placed a

printk("EIP: 0x%016lx, 0x%016lx\n", v->arch.user_regs.eip,
rsp->data.regs.x86.rip);

at the top of vm_event_set_registers(), so I could see what the old
value is (v->arch.user_regs.eip) vs. the new value (rsp->data.regs.x86.rip).

I'm also logging these in my application, the difference there being
that the old value is the value that came with the vm_event request,
which has been obtained with guest_cpu_user_regs()->eip (where in
vm_event_set_registers() we set v->arch.user_regs.eip, since v != current).

Here's a short test run, with xl dmesg:

(XEN) [  395.739543] EIP: 0xfffff80001be5957, 0xfffff80001be595b
(XEN) [  409.795311] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  416.675023] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  421.475567] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  428.275125] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  435.507009] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  441.318224] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  445.514807] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  452.539190] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  454.762810] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  459.538651] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  462.027222] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  481.770470] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  483.298493] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  486.522344] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  491.042325] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  494.874468] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  497.450765] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  500.562738] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  509.179754] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  510.826236] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  512.106206] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  518.658092] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  520.450156] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  521.882088] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  523.250092] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  524.577987] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  525.962042] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  527.353942] EIP: 0xfffff80001be4812, 0xfffff80001be4812
(XEN) [  528.714089] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  530.065994] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  531.357762] EIP: 0xfffff80001be4812, 0xfffff80001be4812
(XEN) [  532.594016] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  533.849886] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  535.145879] EIP: 0xfffff80001be4812, 0xfffff80001be4812
(XEN) [  536.546846] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  537.905756] EIP: 0xfffff80001be480f, 0xfffff80001be4812
(XEN) [  539.209454] EIP: 0xfffff80001be4812, 0xfffff80001be4812

and the corresponding part in the userspace application's log:

GET EIP: 0xfffff80001be5957 SET EIP: 0xfffff80001be595b
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812
GET EIP: 0xfffff80001be480f SET EIP: 0xfffff80001be4812

As you can see, they're different (and correct in the application log),
but (for example for the last one) they both already have the same value
in the hypervisor. So it would appear that guest_cpu_user_regs()->eip !=
v->arch.user_regs.eip at that point (and likely even more state than
that differs there).

These are EPT fault events, all of them, and I just reply with
VM_EVENT_FLAG_SET_REGISTERS to them here, and nothing else.


Thanks,
Razvan

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