x86/boot/64: Use RIP_REL_REF() to assign 'phys_base'

'phys_base' is assigned from code that executes from a 1:1 mapping so it
cannot use a plain access from C. Replace the use of fixup_pointer()
with RIP_REL_REF(), which is better and simpler.

While at it, move the assignment to before the addition of the SME mask
so there is no need to subtract it again, and drop the unnecessary
addition ('phys_base' is statically initialized to 0x0)

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240221113506.2565718-20-ardb+git@google.com

arch/x86/kernel/head64.c

@@ -192,6 +192,7 @@ unsigned long __head __startup_64(unsigned long physaddr,
 	 * and the address I am actually running at.
 	 */
 	load_delta = physaddr - (unsigned long)(_text - __START_KERNEL_map);
+	RIP_REL_REF(phys_base) = load_delta;
 
 	/* Is the address not 2M aligned? */
 	if (load_delta & ~PMD_MASK)
@@ -301,12 +302,6 @@ unsigned long __head __startup_64(unsigned long physaddr,
 	for (; i < PTRS_PER_PMD; i++)
 		pmd[i] &= ~_PAGE_PRESENT;
 
-	/*
-	 * Fixup phys_base - remove the memory encryption mask to obtain
-	 * the true physical address.
-	 */
-	*fixup_long(&phys_base, physaddr) += load_delta - sme_get_me_mask();
-
 	return sme_postprocess_startup(bp, pmd);
 }