For root partition the VP assist pages are pre-determined by the
hypervisor. The root kernel is not allowed to change them to
different locations. And thus, we are getting below stack as in
current implementation root is trying to perform write to specific
MSR.

    [ 2.778197] unchecked MSR access error: WRMSR to 0x40000073 (tried to write 0x0000000145ac5001) at rIP: 0xffffffff810c1084 (native_write_msr+0x4/0x30)
    [ 2.784867] Call Trace:
    [ 2.791507] hv_cpu_init+0xf1/0x1c0
    [ 2.798144] ? hyperv_report_panic+0xd0/0xd0
    [ 2.804806] cpuhp_invoke_callback+0x11a/0x440
    [ 2.811465] ? hv_resume+0x90/0x90
    [ 2.818137] cpuhp_issue_call+0x126/0x130
    [ 2.824782] __cpuhp_setup_state_cpuslocked+0x102/0x2b0
    [ 2.831427] ? hyperv_report_panic+0xd0/0xd0
    [ 2.838075] ? hyperv_report_panic+0xd0/0xd0
    [ 2.844723] ? hv_resume+0x90/0x90
    [ 2.851375] __cpuhp_setup_state+0x3d/0x90
    [ 2.858030] hyperv_init+0x14e/0x410
    [ 2.864689] ? enable_IR_x2apic+0x190/0x1a0
    [ 2.871349] apic_intr_mode_init+0x8b/0x100
    [ 2.878017] x86_late_time_init+0x20/0x30
    [ 2.884675] start_kernel+0x459/0x4fb
    [ 2.891329] secondary_startup_64_no_verify+0xb0/0xbb

Since the hypervisor already provides the VP assist pages for root
partition, we need to memremap the memory from hypervisor for root
kernel to use. The mapping is done in hv_cpu_init during bringup and is
unmapped in hv_cpu_die during teardown.
Signed-off-by: Praveen Kumar <kumarpraveen@linux.microsoft.com>
Reviewed-by: Sunil Muthuswamy <sunilmut@microsoft.com>
Link: https://lore.kernel.org/r/20210731120519.17154-1-kumarpraveen@linux.microsoft.comSigned-off-by: Wei Liu <wei.liu@kernel.org>