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

[Fabian Grünbichler]

On Tue, Jan 09, 2018 at 05:45:52PM +0100, Michael Rasmussen wrote:
> On Tue, 9 Jan 2018 16:31:40 +0100
> Stefan Priebe - Profihost AG <s.priebe at profihost.ag> wrote:
> 
> > 
> > Thanks! I would love to see it as a default for the specific qemu cpu
> > models.
> > 
> > I mean we know exactly that sandybridge, ivybridge, ... support it or not?
> > 
> Is this an Intel only cpu flag?
> 
> lscpu |grep Model
> Model:                 2
> Model name:            AMD Opteron(tm) Processor 3350 HE
> 
> grep pcid /proc/cpuinfo
> 

yes. it's an Intel CPU feature, and only needed to reduce the
performance impact of KPTI, which is a mitigation for a bug affecting
Intel CPUs (and some ARM). since KPTI is disabled unless manually forced
on AMD CPUs, the fact that there is no PCID does not matter there ;)

similar changes with other CPU features will be needed for the Spectre
mitigation patches which will probably be finalized upstream soon, those
will affect AMD as well.