target/i386: sev: Use ram_block_discard_disable()

AMD SEV will pin all guest memory, mark discarding of RAM broken. At the
time this is called, we cannot have anyone active that relies on discards
to work properly - let's still implement error handling.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200626072248.78761-8-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

target/i386/sev.c

@@ -680,6 +680,12 @@ sev_guest_init(const char *id)
     uint32_t host_cbitpos;
     struct sev_user_data_status status = {};
 
+    ret = ram_block_discard_disable(true);
+    if (ret) {
+        error_report("%s: cannot disable RAM discard", __func__);
+        return NULL;
+    }
+
     sev = lookup_sev_guest_info(id);
     if (!sev) {
         error_report("%s: '%s' is not a valid '%s' object",
@@ -751,6 +757,7 @@ sev_guest_init(const char *id)
     return sev;
 err:
     sev_guest = NULL;
+    ram_block_discard_disable(false);
     return NULL;
 }