x86/sev: Add a helper for the PVALIDATE instruction

An SNP-active guest uses the PVALIDATE instruction to validate or
rescind the validation of a guest page’s RMP entry. Upon completion, a
return code is stored in EAX and rFLAGS bits are set based on the return
code. If the instruction completed successfully, the carry flag (CF)
indicates if the content of the RMP were changed or not.

See AMD APM Volume 3 for additional details.
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Venu Busireddy <venu.busireddy@oracle.com>
Link: https://lore.kernel.org/r/20220307213356.2797205-14-brijesh.singh@amd.com

arch/x86/include/asm/sev.h

@@ -60,6 +60,9 @@ extern void vc_no_ghcb(void);
 extern void vc_boot_ghcb(void);
 extern bool handle_vc_boot_ghcb(struct pt_regs *regs);
 
+/* Software defined (when rFlags.CF = 1) */
+#define PVALIDATE_FAIL_NOUPDATE		255
+
 #ifdef CONFIG_AMD_MEM_ENCRYPT
 extern struct static_key_false sev_es_enable_key;
 extern void __sev_es_ist_enter(struct pt_regs *regs);
@@ -87,12 +90,30 @@ extern enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb,
 					  struct es_em_ctxt *ctxt,
 					  u64 exit_code, u64 exit_info_1,
 					  u64 exit_info_2);
+static inline int pvalidate(unsigned long vaddr, bool rmp_psize, bool validate)
+{
+	bool no_rmpupdate;
+	int rc;
+
+	/* "pvalidate" mnemonic support in binutils 2.36 and newer */
+	asm volatile(".byte 0xF2, 0x0F, 0x01, 0xFF\n\t"
+		     CC_SET(c)
+		     : CC_OUT(c) (no_rmpupdate), "=a"(rc)
+		     : "a"(vaddr), "c"(rmp_psize), "d"(validate)
+		     : "memory", "cc");
+
+	if (no_rmpupdate)
+		return PVALIDATE_FAIL_NOUPDATE;
+
+	return rc;
+}
 #else
 static inline void sev_es_ist_enter(struct pt_regs *regs) { }
 static inline void sev_es_ist_exit(void) { }
 static inline int sev_es_setup_ap_jump_table(struct real_mode_header *rmh) { return 0; }
 static inline void sev_es_nmi_complete(void) { }
 static inline int sev_es_efi_map_ghcbs(pgd_t *pgd) { return 0; }
+static inline int pvalidate(unsigned long vaddr, bool rmp_psize, bool validate) { return 0; }
 #endif
 
 #endif