x86/tdx: Use ReportFatalError to report missing SEPT_VE_DISABLE

Linux TDX guests require that the SEPT_VE_DISABLE "attribute" be set.
If it is not set, the kernel is theoretically required to handle
exceptions anywhere that kernel memory is accessed, including places
like NMI handlers and in the syscall entry gap.

Rather than even try to handle these exceptions, the kernel refuses to
run if SEPT_VE_DISABLE is unset.

However, the SEPT_VE_DISABLE detection and refusal code happens very
early in boot, even before earlyprintk runs.  Calling panic() will
effectively just hang the system.

Instead, call a TDX-specific panic() function.  This makes a very simple
TDVMCALL which gets a short error string out to the hypervisor without
any console infrastructure.

Use TDG.VP.VMCALL<ReportFatalError> to report the error. The hypercall
can encode message up to 64 bytes in eight registers.

[ dhansen: tweak comment and remove while loop brackets. ]

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20230126221159.8635-6-kirill.shutemov%40linux.intel.com

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