x86/boot: Fix SEV boot failure from change to __PHYSICAL_MASK_SHIFT

In arch/x86/boot/compressed/kaslr_64.c, CONFIG_AMD_MEM_ENCRYPT support was
initially #undef'd to support SME with minimal effort.  When support for
SEV was added, the #undef remained and some minimal support for setting the
encryption bit was added for building identity mapped pagetable entries.

Commit b83ce5ee9147 ("x86/mm/64: Make __PHYSICAL_MASK_SHIFT always 52")
changed __PHYSICAL_MASK_SHIFT from 46 to 52 in support of 5-level paging.
This change resulted in SEV guests failing to boot because the encryption
bit was no longer being automatically masked out.  The compressed boot
path now requires sme_me_mask to be defined in order for the pagetable
functions, such as pud_present(), to properly mask out the encryption bit
(currently bit 47) when evaluating pagetable entries.

Add an sme_me_mask variable in arch/x86/boot/compressed/mem_encrypt.S,
which is set when SEV is active, delete the #undef CONFIG_AMD_MEM_ENCRYPT
from arch/x86/boot/compressed/kaslr_64.c and use sme_me_mask when building
the identify mapped pagetable entries.

Fixes: b83ce5ee9147 ("x86/mm/64: Make __PHYSICAL_MASK_SHIFT always 52")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lkml.kernel.org/r/20180327220711.8702.55842.stgit@tlendack-t1.amdoffice.net

diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S
index 54f5f66..eaa843a 100644
--- a/arch/x86/boot/compressed/mem_encrypt.S
+++ b/arch/x86/boot/compressed/mem_encrypt.S
@@ -88,9 +88,7 @@
 ENDPROC(get_sev_encryption_bit)
 
 	.code64
-ENTRY(get_sev_encryption_mask)
-	xor	%rax, %rax
-
+ENTRY(set_sev_encryption_mask)
 #ifdef CONFIG_AMD_MEM_ENCRYPT
 	push	%rbp
 	push	%rdx
@@ -101,9 +99,7 @@
 	testl	%eax, %eax
 	jz	.Lno_sev_mask
 
-	xor	%rdx, %rdx
-	bts	%rax, %rdx		/* Create the encryption mask */
-	mov	%rdx, %rax		/* ... and return it */
+	bts	%rax, sme_me_mask(%rip)	/* Create the encryption mask */
 
 .Lno_sev_mask:
 	movq	%rbp, %rsp		/* Restore original stack pointer */
@@ -112,9 +108,16 @@
 	pop	%rbp
 #endif
 
+	xor	%rax, %rax
 	ret
-ENDPROC(get_sev_encryption_mask)
+ENDPROC(set_sev_encryption_mask)
 
 	.data
 enc_bit:
 	.int	0xffffffff
+
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+	.balign	8
+GLOBAL(sme_me_mask)
+	.quad	0
+#endif