[Libguestfs] [v2v PATCH v2] output/create_libvirt_xml: relax VCPU feature checking for "qemu64"

[Laszlo Ersek]

When the source domain doesn't specify a VCPU model ("s_cpu_model" is
None), and the guest OS is assumed to work with the default VCPU model
("gcaps_default_cpu" is true), we don't output any <cpu> element. In that
case, libvirtd augments the domain config with:

  [1] <cpu mode='custom' match='exact' check='none'>
        <model fallback='forbid'>qemu64</model>
      </cpu>

where the @check='none' attribute ensures that the converted domain will
be launched, for example, on an Intel host, despite the "qemu64" VCPU
model containing AMD-only feature flags such as "svm".

However, if the source domain explicitly specifies the "qemu64" model
(mostly seen with "-i libvirt -ic qemu://..."), we presently output

  [2] <cpu match='minimum'>
        <model fallback='allow'>qemu64</model>
      </cpu>

which libvirtd completes as

  [3] <cpu mode='custom' match='minimum' check='partial'>
        <model fallback='allow'>qemu64</model>
      </cpu>

In [3], cpu/@match='minimum' and cpu/model/@fallback='allow' are both
laxer than @match='exact' and @fallback='forbid', respectively, in [1].

However, cpu/@check='partial' in [3] is stricter than @check='none' in
[1]; it causes libvirtd to catch the "svm" feature flag on an Intel host,
and prevents the converted domain from starting.

The "qemu64" VCPU model is supposed to run on every possible host
<https://gitlab.com/qemu-project/qemu/-/blob/master/docs/system/cpu-models-x86.rst.inc>,
therefore make an exception for the explicitly specified "qemu64" VCPU
model, and generate the @check='none' attribute.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2107503
Signed-off-by: Laszlo Ersek <lersek at redhat.com>
---

Notes:
    v2:
    - make the attribute dependent on "qemu64" [Rich]
    - retrofit the commit message
    
    v1:
    - https://listman.redhat.com/archives/libguestfs/2022-July/029504.html

 output/create_libvirt_xml.ml | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/output/create_libvirt_xml.ml b/output/create_libvirt_xml.ml
index 531a4f75bf3e..bd01304df674 100644
--- a/output/create_libvirt_xml.ml
+++ b/output/create_libvirt_xml.ml
@@ -192,6 +192,8 @@ let create_libvirt_xml ?pool source inspect
            List.push_back cpu_attrs ("mode", "host-passthrough");
      | Some model ->
          List.push_back cpu_attrs ("match", "minimum");
+         if model = "qemu64" then
+           List.push_back cpu_attrs ("check", "none");
          (match source.s_cpu_vendor with
           | None -> ()
           | Some vendor ->
-- 
2.19.1.3.g30247aa5d201