On Thu, Sep 28, 2017 at 09:21:41AM +0100, Daniel P. Berrange wrote:
On Thu, Sep 21, 2017 at 01:14:04PM -0400, Laine Stump wrote:
On 09/19/2017 03:37 PM, Eduardo Habkost wrote:
Cache mode=passthrough can result in a broken cache topology if the domain topology is not exactly the same as the host topology. Warn about that in the documentation.
Bug report for reference: https://bugzilla.redhat.com/show_bug.cgi?id=1184125
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com> --- docs/formatdomain.html.in | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in index 57ec2ff34..9c21892f3 100644 --- a/docs/formatdomain.html.in +++ b/docs/formatdomain.html.in @@ -1478,7 +1478,9 @@
<dt><code>passthrough</code></dt> <dd>The real CPU cache data reported by the host CPU will be - passed through to the virtual CPU.</dd> + passed through to the virtual CPU. Using this mode is not + recommended unless the domain CPU and NUMA topology is exactly + the same as the host CPU and NUMA topology.</dd>
To me this sounds like it should be forbidden by libvirt, rather than just documented as "bad". (I haven't followed any previous discussion on the topic though, so maybe I'm over-reacting).
In high performance setups, people pin guest vCPUs to host pCPUs and set the vCPU topology to match the host pCPU topology they've pinned to. So ohaving a cache mode that matches this topology is just fine. It simply isn't something you want as a default for the more typical floating vCPUs scenarios.
So, should this patch be applied? -- Eduardo