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