[PATCH 3/4] i386/sev: Update checks and information related to reduced-phys-bits

[Tom Lendacky <thomas.lendacky@xxxxxxx>]

The value of the reduced-phys-bits parameter is propogated to the CPUID
information exposed to the guest. Update the current validation check to
account for the size of the CPUID field (6-bits), ensuring the value is
in the range of 1 to 63.

Maintain backward compatibility, to an extent, by allowing a value greater
than 1 (so that the previously documented value of 5 still works), but not
allowing anything over 63.

Fixes: d8575c6c02 ("sev/i386: add command to initialize the memory encryption context")
Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
---
 target/i386/sev.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/target/i386/sev.c b/target/i386/sev.c
index 32f7dbac4e..78c2d37eba 100644
--- a/target/i386/sev.c
+++ b/target/i386/sev.c
@@ -932,15 +932,26 @@ int sev_kvm_init(ConfidentialGuestSupport *cgs, Error **errp)
     host_cpuid(0x8000001F, 0, NULL, &ebx, NULL, NULL);
     host_cbitpos = ebx & 0x3f;
 
+    /*
+     * The cbitpos value will be placed in bit positions 5:0 of the EBX
+     * register of CPUID 0x8000001F. No need to verify the range as the
+     * comparison against the host value accomplishes that.
+     */
     if (host_cbitpos != sev->cbitpos) {
         error_setg(errp, "%s: cbitpos check failed, host '%d' requested '%d'",
                    __func__, host_cbitpos, sev->cbitpos);
         goto err;
     }
 
-    if (sev->reduced_phys_bits < 1) {
-        error_setg(errp, "%s: reduced_phys_bits check failed, it should be >=1,"
-                   " requested '%d'", __func__, sev->reduced_phys_bits);
+    /*
+     * The reduced-phys-bits value will be placed in bit positions 11:6 of
+     * the EBX register of CPUID 0x8000001F, so verify the supplied value
+     * is in the range of 1 to 63.
+     */
+    if (sev->reduced_phys_bits < 1 || sev->reduced_phys_bits > 63) {
+        error_setg(errp, "%s: reduced_phys_bits check failed,"
+                   " it should be in the range of 1 to 63, requested '%d'",
+                   __func__, sev->reduced_phys_bits);
         goto err;
     }
 
-- 
2.37.3