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

[Stefan Priebe - Profihost AG]

Am 09.01.2018 um 16:18 schrieb 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).

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?

Greets,
Stefan