x86/sev: Make the VMPL0 checking more straight forward

Currently, the enforce_vmpl0() function uses a set argument when modifying
the VMPL1 permissions used to test for VMPL0. If the guest is not running
at VMPL0, the guest self-terminates.

The function is just a wrapper for a fixed RMPADJUST function. Eliminate
the function and perform the RMPADJUST directly.

No functional change.
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/ed01ddf04bfb475596b24b634fd26cffaa85173a.1713974291.git.thomas.lendacky@amd.com

arch/x86/boot/compressed/sev.c

@@ -335,26 +335,6 @@ void do_boot_stage2_vc(struct pt_regs *regs, unsigned long exit_code)
 		sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SEV_ES_GEN_REQ);
 }
 
-static void enforce_vmpl0(void)
-{
-	u64 attrs;
-	int err;
-
-	/*
-	 * RMPADJUST modifies RMP permissions of a lesser-privileged (numerically
-	 * higher) privilege level. Here, clear the VMPL1 permission mask of the
-	 * GHCB page. If the guest is not running at VMPL0, this will fail.
-	 *
-	 * If the guest is running at VMPL0, it will succeed. Even if that operation
-	 * modifies permission bits, it is still ok to do so currently because Linux
-	 * SNP guests are supported only on VMPL0 so VMPL1 or higher permission masks
-	 * changing is a don't-care.
-	 */
-	attrs = 1;
-	if (rmpadjust((unsigned long)&boot_ghcb_page, RMP_PG_SIZE_4K, attrs))
-		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_NOT_VMPL0);
-}
-
 /*
  * SNP_FEATURES_IMPL_REQ is the mask of SNP features that will need
  * guest side implementation for proper functioning of the guest. If any
@@ -588,7 +568,20 @@ void sev_enable(struct boot_params *bp)
 		if (!(get_hv_features() & GHCB_HV_FT_SNP))
 			sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SNP_UNSUPPORTED);
 
-		enforce_vmpl0();
+		/*
+		 * Enforce running at VMPL0.
+		 *
+		 * RMPADJUST modifies RMP permissions of a lesser-privileged (numerically
+		 * higher) privilege level. Here, clear the VMPL1 permission mask of the
+		 * GHCB page. If the guest is not running at VMPL0, this will fail.
+		 *
+		 * If the guest is running at VMPL0, it will succeed. Even if that operation
+		 * modifies permission bits, it is still ok to do so currently because Linux
+		 * SNP guests running at VMPL0 only run at VMPL0, so VMPL1 or higher
+		 * permission mask changes are a don't-care.
+		 */
+		if (rmpadjust((unsigned long)&boot_ghcb_page, RMP_PG_SIZE_4K, 1))
+			sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_NOT_VMPL0);
 	}
 
 	if (snp && !(sev_status & MSR_AMD64_SEV_SNP_ENABLED))