x86/microcode/AMD: Handle the case of no BIOS microcode

Machines can be shipped without any microcode in the BIOS. Which means,
the microcode patch revision is 0.

Handle that gracefully.

Fixes: 94838d230a6c ("x86/microcode/AMD: Use the family,model,stepping encoded in the patch ID")
Reported-by: Vítek Vávra <[e-mail]>
Signed-off-by: Borislav Petkov (AMD) <[e-mail]>
Cc: <[e-mail]>

arch/x86/kernel/cpu/microcode/amd.c

···
 		return 1;
 }
 
+static u32 cpuid_to_ucode_rev(unsigned int val)
+{
+	union zen_patch_rev p = {};
+	union cpuid_1_eax c;
+
+	c.full = val;
+
+	p.stepping  = c.stepping;
+	p.model     = c.model;
+	p.ext_model = c.ext_model;
+	p.ext_fam   = c.ext_fam;
+
+	return p.ucode_rev;
+}
+
 static bool need_sha_check(u32 cur_rev)
 {
+	if (!cur_rev) {
+		cur_rev = cpuid_to_ucode_rev(bsp_cpuid_1_eax);
+		pr_info_once("No current revision, generating the lowest one: 0x%x\n", cur_rev);
+	}
+
 	switch (cur_rev >> 8) {
 	case 0x80012: return cur_rev <= 0x800126f; break;
 	case 0x80082: return cur_rev <= 0x800820f; break;
···
 
 	n.equiv_cpu = equiv_cpu;
 	n.patch_id  = uci->cpu_sig.rev;
-
-	WARN_ON_ONCE(!n.patch_id);
 
 	list_for_each_entry(p, &microcode_cache, plist)
 		if (patch_cpus_equivalent(p, &n, false))