Help with Understanding vcpu xstate restore error during vm migration

[Fonyuy-Asheri Caleb <fonyuy-asheri.caleb@xxxxxxxx>]

Hello, 

I am trying to understand the causes of the vcpu xstate restore error during live migration.

I get the following error during live migration: 

xc: error: Failed to set vcpu0's xsave info (22 = Invalid argument): Internal error

I was able to locate the failure point to the file xen/arch/x86/domctl.c  with the following check.

if ( evc->size < PV_XSAVE_HDR_SIZE ||

evc->size > PV_XSAVE_SIZE(xfeature_mask) )

goto vcpuextstate_out;

I know this is related to the number of xstates handled by the source server. Please can 

someone explain to me how these states are computed? 

I earlier thought it was simply the number xsave dependent features on the CPU but it seems

to be more than that. 

Thanks in advance. 

Caleb