Re: VM_EVENT_FLAG_SET_REGISTERS and sync_vcpu_execstate()

[Andrew Cooper <andrew.cooper3@citrix.com>]

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.

~Andrew

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