From fbb053fc71c0c072acb3fbf6e5fbbfc3b0667fd2 Mon Sep 17 00:00:00 2001
From: "Richard W.M. Jones" <rjones@redhat.com>
Date: Thu, 28 Jan 2021 12:20:49 +0000
Subject: [PATCH] appliance: Use -cpu max.
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QEMU has a newish feature (from about 2017 / qemu 2.9) called -cpu max
which is supposed to select the best CPU, ideal for libguestfs.

After this change, on x86-64:

               KVM                          TCG

Direct         -cpu max                     -cpu max
(non-libvirt)

Libvirt   <cpu mode="host-passthrough">     <cpu mode="host-model">
            <model fallback="allow"/>         <model fallback="allow"/>
          </cpu>                            </cpu>

Thanks: Daniel Berrangé
(cherry picked from commit 30f74f38bd6e42e783ba80895f4d6826abddd417)
---
 lib/appliance-cpu.c  | 16 ++++++++--------
 lib/launch-libvirt.c |  9 +++++++++
 2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/lib/appliance-cpu.c b/lib/appliance-cpu.c
index 5ef9f5c72..54ac6e2e3 100644
--- a/lib/appliance-cpu.c
+++ b/lib/appliance-cpu.c
@@ -38,6 +38,11 @@
  *
  * The literal string C<"host"> means use C<-cpu host>.
  *
+ * =item C<"max">
+ *
+ * The literal string C<"max"> means use C<-cpu max> (the best
+ * possible).  This requires awkward translation for libvirt.
+ *
  * =item some string
  *
  * Some string such as C<"cortex-a57"> means use C<-cpu cortex-a57>.
@@ -80,14 +85,9 @@ guestfs_int_get_cpu_model (int kvm)
   /* See discussion in https://bugzilla.redhat.com/show_bug.cgi?id=1605071 */
   return NULL;
 #else
-  /* On most architectures, it is faster to pass the CPU host model to
-   * the appliance, allowing maximum speed for things like checksums
-   * and encryption.  Only do this with KVM.  It is broken in subtle
-   * ways on TCG, and fairly pointless when you're emulating anyway.
+  /* On most architectures we can use "max" to get the best possible CPU.
+   * For recent qemu this should work even on TCG.
    */
-  if (kvm)
-    return "host";
-  else
-    return NULL;
+  return "max";
 #endif
 }
diff --git a/lib/launch-libvirt.c b/lib/launch-libvirt.c
index f6bb39d99..e3ff1ffe0 100644
--- a/lib/launch-libvirt.c
+++ b/lib/launch-libvirt.c
@@ -1169,6 +1169,15 @@ construct_libvirt_xml_cpu (guestfs_h *g,
           attribute ("fallback", "allow");
         } end_element ();
       }
+      else if (STREQ (cpu_model, "max")) {
+        if (params->data->is_kvm)
+          attribute ("mode", "host-passthrough");
+        else
+          attribute ("mode", "host-model");
+        start_element ("model") {
+          attribute ("fallback", "allow");
+        } end_element ();
+      }
       else
         single_element ("model", cpu_model);
     } end_element ();
-- 
2.31.1