Subject: x86/cpu-policy: Fix handling of leaf 0x80000021
From: Andrew Cooper <[email-masked]>
Date: Tue, 1 Jul 2025 11:33:41 +0100

When support was originally introduced, ebx, ecx and edx were reserved and
should have been zeroed in recalculate_misc() to avoid leaking into guests.

Since then, fields have been added into ebx.  Guests can't load microcode, so
shouldn't see ucode_size, and while in principle we do want to support larger
RAP sizes in guests, virtualising this for guests depends on AMD procuding any
official documentation for ERAPS, which is long overdue and with no ETA.

This patch will cause a difference in guests on Zen5 CPUs, but as the main
ERAPS feature is hidden, guests should be ignoring the rap_size field too.

Fixes: e9b4fe263649 ("x86/cpuid: support LFENCE always serialising CPUID bit")
Signed-off-by: Andrew Cooper <[email-masked]>
Reviewed-by: Jan Beulich <[email-masked]>
(cherry picked from commit 10dc35c516f7b9224590a7a4e2722bbfd70fa87a)

diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
index 42bd039e7c2e..8f006fe08acb 100644
--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -340,6 +340,9 @@ static void recalculate_misc(struct cpu_policy *p)
         p->extd.raw[0x1e] = EMPTY_LEAF; /* TopoExt APIC ID/Core/Node */
         p->extd.raw[0x1f] = EMPTY_LEAF; /* SEV */
         p->extd.raw[0x20] = EMPTY_LEAF; /* Platform QoS */
+        p->extd.raw[0x21].b = 0;
+        p->extd.raw[0x21].c = 0;
+        p->extd.raw[0x21].d = 0;
         break;
     }
 }
diff --git a/xen/include/xen/lib/x86/cpu-policy.h b/xen/include/xen/lib/x86/cpu-policy.h
index 6d5e9edd269b..ba29bfe9b414 100644
--- a/xen/include/xen/lib/x86/cpu-policy.h
+++ b/xen/include/xen/lib/x86/cpu-policy.h
@@ -324,7 +324,10 @@ struct cpu_policy
                 uint32_t e21a;
                 struct { DECL_BITFIELD(e21a); };
             };
-            uint32_t /* b */:32, /* c */:32, /* d */:32;
+            uint16_t ucode_size; /* Units of 16 bytes */
+            uint8_t  rap_size;   /* Units of 8 entries */
+            uint8_t  :8;
+            uint32_t /* c */:32, /* d */:32;
         };
     } extd;