Re: [Libguestfs] [v2v PATCH] output/create_libvirt_xml: generate @check='none' CPU attribute

We currently don't generate any @check attribute for the /domain/cpu element, which causes the following libvirtd behavior <https://libvirt.org/formatdomain.html#cpu-model-and-topology>;: > Once the domain starts, libvirt will automatically change the check > attribute to the best supported value to ensure the virtual CPU does not > change when the domain is migrated to another host Vera Wu reports that in practice, at least when the source CPU model is explicitly "qemu64", libvirtd sets @check='partial'. That's defined as:

> Libvirt will check the guest CPU specification before starting a domain This is a problem: the default "qemu64" CPU model exposes the SVM CPU flag, and that's unsupportable on Intel hosts. SVM is the AMD counterpart of VT-x; IOW, the flag effectively advertizes AMD-specific nesting to guests. With @check='partial', libvirt prevents the converted domain from starting on Intel hosts; but with @check='none', > Libvirt does no checking and it is up to the hypervisor to refuse to > start the domain if it cannot provide the requested CPU. With QEMU this > means no checking is done at all since the default behavior of QEMU is > to emit warnings, but start the domain anyway. We don't care about the migratability of the converted domain, so relax the libvirtd checks, by generating the @check='none' attribute.

Consider adding @check='none' in two cases: (1) When the source domain specifies a CPU model. Generating @check='none' in this case fixes the issue reported by Vera. (2) When the source domain does not specify a CPU model, and the guest OS is assumed to work well with the default CPU model. Generating @check='none' in this case is actually a no-op. Going from "no CPU element" to "<cpu check='none'/>" does not change how libvirtd augments the domain config. Namely, (2.1) for x86 guests, we get <cpu mode='custom' match='exact' check='none'> <model fallback='forbid'>qemu64</model> </cpu> either way, (2.2) for aarch64 guests, we get <cpu mode='custom' match='exact' check='none'> <model fallback='forbid'>cortex-a15</model> </cpu> either way. Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2107503 Signed-off-by: Laszlo Ersek <lersek(a)redhat.com&gt; --- output/create_libvirt_xml.ml | 1 + 1 file changed, 1 insertion(+) diff --git a/output/create_libvirt_xml.ml b/output/create_libvirt_xml.ml index 531a4f75bf3e..0343d3194268 100644 --- a/output/create_libvirt_xml.ml +++ b/output/create_libvirt_xml.ml @@ -192,6 +192,7 @@ let create_libvirt_xml ?pool source inspect List.push_back cpu_attrs ("mode", "host-passthrough"); | Some model -> List.push_back cpu_attrs ("match", "minimum"); + List.push_back cpu_attrs ("check", "none");

(match source.s_cpu_vendor with | None -> () | Some vendor -> -- 2.19.1.3.g30247aa5d201 _______________________________________________ Libguestfs mailing list Libguestfs(a)redhat.com https://listman.redhat.com/mailman/listinfo/libguestfs