[pve-devel] cpuflag: pcid needed in guest for good performance after meltdown

[Fabian Grünbichler]

On Tue, Jan 09, 2018 at 02:58:24PM +0100, Fabian Grünbichler wrote:
> On Mon, Jan 08, 2018 at 09:34:57PM +0100, Stefan Priebe - Profihost AG wrote:
> > Hello,
> > 
> > for meltdown mitigation and performance it's important to have the pcid
> > flag passed down to the guest (f.e.
> > https://groups.google.com/forum/m/#!topic/mechanical-sympathy/L9mHTbeQLNU).
> > 
> > My host shows the flag:
> > # grep ' pcid ' /proc/cpuinfo  | wc -l
> > 56
> > 
> > But the guest does not:
> > # grep pcid /proc/cpuinfo
> > #
> > 
> > Guest was started with:
> > -cpu IvyBridge,+kvm_pv_unhalt,+kvm_pv_eoi,enforce,vendor=GenuineIntel
> > 
> > Is this something missing in host kernel or in PVE?
> > 
> > Greets,
> > Stefan
> 
> we are preparing a patch for qemu-server to allow enabling the pcid
> cpuflag on a VM config level (especially since most VMs will run with
> kvm64, where changing the default is not an option!).
> 
> switching it on by default for SandyBridge and co (which are missing it
> in Qemu, but support it technically) will likely happen with the move to
> Qemu 2.11, pending further feedback and review (it is not clear at all
> how various guest OS handle getting the flag changed out under them when
> migrating, and we don't want to integrate specific pinning support in a
> rushed through manner).
> 
> for now you can of course just patch a hardcoded +pcid in to the cpu
> flag list on your hosts if you know your CPUs all support it and
> stop/start your VMs to regain lost performance.
> 

should be on pvetest for 5.x and 4.x shortly - please provide feedback!

patches for exposing it somewhere on the GUI will follow (most likely
tomorrow).