Bug 1732726

I:
On the same host(rhel-alt-7.6):
# /usr/libexec/qemu-kvm -M pseries-rhel7.6.0
VNC server running on ::1:5900

if qemu cli without "sxxm", it will be ok.

II.
on rhelav8.0.1 host:
# /usr/libexec/qemu-kvm -M pseries-rhel7.6.0-sxxm
VNC server running on ::1:5900
qemu-kvm: Requested safe indirect branch capability level not supported by kvm, try cap-ibs=fixed-ibs

# /usr/libexec/qemu-kvm -M pseries-rhel8.0.0
works well.

Hi, Jiri,
could you help to check this issue, thanks

ngu,

I see you set needinfo? for me, but I don't see a question.  What is it you want to know?

(In reply to David Gibson from comment #20)
> ngu,
> 
> I see you set needinfo? for me, but I don't see a question.  What is it you
> want to know?

Hi David,

It's in comment 18, to follow up the action in comment 16. Do you think it's meaningful to add support for parameter 'ibs' in libvirt xm so we can recommend users to use cap-ibs=workaround?

Jiri, update text looks better to me.  One final revision:

I don't think there's any point suggesting different options for the different CPU versions.  I'd just suggest cap-ibs=workaround in all cases.  That should work for both CPUs, and is the currently recommended mitigation for this bug.

(In reply to David Gibson from comment #22)
> Jiri, update text looks better to me.  One final revision:
> 
> I don't think there's any point suggesting different options for the
> different CPU versions.  I'd just suggest cap-ibs=workaround in all cases. 
> That should work for both CPUs, and is the currently recommended mitigation
> for this bug.

Hold on. The doc text reads

  To work around this problem, add one of the following strings to the
  kernel command line of the VM, depending on the CPU firmware:

  * For DD2.2: `pseries-rhel7.6.0-sxxm,cap-ibs=fixed-ibs`
  * For DD2.3: `pseries-rhel7.6.0-sxxm,cap-ibs=workaround`

which is clearly not correct, as it's not the *kernel* command line
that needs tweaking but rather the *QEMU* command line.


Additionally, since we don't support running QEMU directly, we need
to document the solution in a libvirt-compatible format: either by
implementing the flag properly, as suggested earlier by Gu Nini, or
at the very least by providing an equivalent XML snippet such as
(untested)

  <domain type='qemu' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
    <qemu:commandline>
      <qemu:arg value='-machine'/>
      <qemu:arg value='pseries-rhel7.6.0-sxxm,cap-ibs=workaround'/>
    </qemu:commandline>
    ...

Duh.  Good point, I totally missed that Andrea.

Yes, this is about the qemu command line, not the kernel command line.

Jiři, yes I think we're going to need the XML workaround instead.

I guess for now we can just use the hack that puts the qemu command line argument into the XML, though longer term it might be worth adding proper support for the parameter to libvirt.  We shouldn't need to repeat the machine type name in there, though.  So we could just use:

<domain type='qemu' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  <qemu:commandline>
    <qemu:arg value='-machine'/>
    <qemu:arg value='cap-ibs=workaround'/>
  </qemu:commandline>
</domain>

According to what David said in Comment 22 and Comment 24, the XML
snippet doesn't need to be different for the two CPU models and the
machine type doesn't need to be included, so we should be able to
apply the following diff:

--- doctext.orig	2019-12-12 09:27:19.132159938 +0100
+++ doctext	2019-12-12 09:27:46.482350262 +0100
@@ -10,22 +10,10 @@
 
 To work around this problem, configure the virtual machine's XML configuration as follows:
 
-* For DD2.2:
-
-----
-  <domain type='qemu' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
-    <qemu:commandline>
-      <qemu:arg value='-machine'/>
-      <qemu:arg value='pseries-rhel7.6.0-sxxm,cap-ibs=fixed-ibs'/>
-    </qemu:commandline>
-----
-
-* For DD2.3:
-
 ----
   <domain type='qemu' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
     <qemu:commandline>
       <qemu:arg value='-machine'/>
-      <qemu:arg value='pseries-rhel7.6.0-sxxm,cap-ibs=workaround'/>
+      <qemu:arg value='cap-ibs=workaround'/>
     </qemu:commandline>
 ----

Understood and updated :-) Thank you again folks!

After evaluating this issue, there are no plans to address it further or fix it in an upcoming release.  Therefore, it is being closed.  If plans change such that this issue will be fixed in an upcoming release, then the bug can be reopened.