x86/tboot: avoid recursive fault in early boot panic with tboot

x86/tboot: avoid recursive fault in early boot panic with tboot

If panic is called before init_idle_domain on a tboot-launched system,
then Xen recursively faults in write_ptbase as seen below.

(XEN)    [<ffff82d080286690>] write_ptbase+0/0x10
(XEN)    [<ffff82d0802c4c3b>] tboot_shutdown+0x6b/0x260
(XEN)    [<ffff82d08029ddac>] machine_restart+0xac/0x2d0
(XEN)    [<ffff82d080286690>] write_ptbase+0/0x10
(XEN)    [<ffff82d0802446c1>] panic+0x111/0x120
(XEN)    [<ffff82d0802a51c1>] do_general_protection+0x171/0x1f0
(XEN)    [<ffff82d080287a82>] mm.c#virt_to_xen_l2e+0x12/0x1c0
(XEN)    [<ffff82d080354720>] x86_64/entry.S#handle_exception_saved+0x66/0xa4
(XEN)    [<ffff82d080286690>] write_ptbase+0/0x10
(XEN)    [<ffff82d0802c4c3b>] tboot_shutdown+0x6b/0x260
(XEN)    [<ffff82d08029ddac>] machine_restart+0xac/0x2d0
(XEN)    [<ffff82d0802446c1>] panic+0x111/0x120
(XEN)    [<ffff82d0803c11a0>] setup.c#bootstrap_map+0/0x11a
(XEN)    [<ffff82d0803b82a0>] flask_op.c#parse_flask_param+0/0xb0
(XEN)    [<ffff82d0803c11a0>] setup.c#bootstrap_map+0/0x11a
(XEN)    [<ffff82d0803b6f6c>] xsm_multiboot_init+0x7c/0xb0
(XEN)    [<ffff82d0803c34bb>] __start_xen+0x1d2b/0x2da0
(XEN)    [<ffff82d0802000f3>] __high_start+0x53/0x60

idle_vcpu[0] is still poisoned with INVALID_VCPU, so write_ptbase faults
dereferencing the pointer.  This fault calls panic and recurses through
the same code path.

If tboot_shutdown is called while idle_vcpu[0] == INVALID_VCPU, then we
are still operating with the initial page tables.  Therefore changing
page tables with write_ptbase is unnecessary.

An easy way to reproduce this is to use tboot to launch an XSM-enabled
Xen without an XSM policy.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>