[Qemu-devel] [PATCH v2] target/i386: sev: add 'sev-max-guests' field to 'query-sev-capabilities'

There are limited numbers of the SEV guests that can be run concurrently.
A management applications may need to know this limit so that it can place
SEV VMs on hosts which have suitable resources available.

Currently, this limit is not exposed to the application. Add a new
'sev-max-guest' field in the query-sev-capabilities to provide this
information.

Cc: Paolo Bonzini <pbonz...@redhat.com>
Cc: Markus Armbruster <arm...@redhat.com>
Cc: Eric Blake <ebl...@redhat.com>
Cc: Daniel P. Berrangé <berra...@redhat.com>
Cc: Laszlo Ersek <ler...@redhat.com>
Cc: Erik Skultety <eskul...@redhat.com>
Cc: Tom Lendacky <thomas.lenda...@amd.com>
Signed-off-by: Brijesh Singh <brijesh.si...@amd.com>
---

changes since v1:
 - document the new field and add (since 4.1) annotation.

 qapi/target.json  | 9 +++++++--
 target/i386/sev.c | 9 +++++++--
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/qapi/target.json b/qapi/target.json
index 1d4d54b600..8cd4fc7919 100644
--- a/qapi/target.json
+++ b/qapi/target.json
@@ -177,13 +177,17 @@
 # @reduced-phys-bits: Number of physical Address bit reduction when SEV is
 #                     enabled
 #
+# @sev-max-guests: maximum number of concurrent SEV guest with SEV-ES disabled
+#                  (since 4.1)
+#
 # Since: 2.12
 ##
 { 'struct': 'SevCapability',
   'data': { 'pdh': 'str',
             'cert-chain': 'str',
             'cbitpos': 'int',
-            'reduced-phys-bits': 'int'},
+            'reduced-phys-bits': 'int',
+            'sev-max-guests': 'int'},
   'if': 'defined(TARGET_I386)' }
 
 ##
@@ -200,7 +204,8 @@
 #
 # -> { "execute": "query-sev-capabilities" }
 # <- { "return": { "pdh": "8CCDD8DDD", "cert-chain": "888CCCDDDEE",
-#                  "cbitpos": 47, "reduced-phys-bits": 5}}
+#                  "cbitpos": 47, "reduced-phys-bits": 5,
+#                  "sev-max-guests" : 15}}
 #
 ##
 { 'command': 'query-sev-capabilities', 'returns': 'SevCapability',
diff --git a/target/i386/sev.c b/target/i386/sev.c
index cd77f6b5d4..6829586fbe 100644
--- a/target/i386/sev.c
+++ b/target/i386/sev.c
@@ -488,7 +488,7 @@ sev_get_capabilities(void)
     guchar *pdh_data = NULL;
     guchar *cert_chain_data = NULL;
     size_t pdh_len = 0, cert_chain_len = 0;
-    uint32_t ebx;
+    uint32_t ebx, ecx, edx;
     int fd;
 
     fd = open(DEFAULT_SEV_DEVICE, O_RDWR);
@@ -507,7 +507,7 @@ sev_get_capabilities(void)
     cap->pdh = g_base64_encode(pdh_data, pdh_len);
     cap->cert_chain = g_base64_encode(cert_chain_data, cert_chain_len);
 
-    host_cpuid(0x8000001F, 0, NULL, &ebx, NULL, NULL);
+    host_cpuid(0x8000001F, 0, NULL, &ebx, &ecx, &edx);
     cap->cbitpos = ebx & 0x3f;
 
     /*
@@ -516,6 +516,11 @@ sev_get_capabilities(void)
      */
     cap->reduced_phys_bits = 1;
 
+    /*
+     * The maximum number of SEV guests with SEV-ES disabled that can run
+     * simultaneously.
+     */
+    cap->sev_max_guests = ecx - edx + 1;
 out:
     g_free(pdh_data);
     g_free(cert_chain_data);
-- 
2.17.1