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

[Alexandre DERUMIER]

they are a discussion on qemu mailing currently about pcid 


>From paolo bonzini:

" 
Note that PCID is still not supported for guests without EPT, so 
this would break ept=0 with recent "-cpu" models. I'm not sure of 
a way to fix it; probably it just has to be documented."

----- Mail original -----
De: "Stefan Priebe, Profihost AG" <s.priebe at profihost.ag>
À: "pve-devel" <pve-devel at pve.proxmox.com>, "aderumier" <aderumier at odiso.com>
Envoyé: Mardi 9 Janvier 2018 08:35:00
Objet: Re: [pve-devel] cpuflag: pcid needed in guest for good performance after meltdown

Am 08.01.2018 um 23:23 schrieb Alexandre DERUMIER: 
> I think it's not exposed in current cpu model 
> 
> it can be enabled with "+pcid" 
> 
> 
> I don't known what it's the best way to add it in proxmox. 
> We could do it with qemu version upgrade, but qemu 2.10 are not ready. 

Yes but that was always bad at least to me. We're limiting ourselfes if 
we can only add new qemu features with a new qemu version. 

What about a version or feature field in the guest config which we only 
update on a fresh vm start. 

Something like this: 
123.conf: 
... 
pve_qemu_version: 2.11-10 
... 

This field is ONLY and ALWAYS updated in a fresh start not on migration. 
This can be easily detected. 

Than we can do stuff like 
if (version_cmp($conf->{pve_qemu_version}, "2.11-11")) { 
# enable pcid flag 
} 

Greets 
Stefan 

> Maybe add new cpumodel with +pcid enabled ? 
> or add code to manage custom cpuflags and add a checkbox in cpu options ? 
> 
> 
> 
> ----- Mail original ----- 
> De: "Stefan Priebe, Profihost AG" <s.priebe at profihost.ag> 
> À: "pve-devel" <pve-devel at pve.proxmox.com> 
> Envoyé: Lundi 8 Janvier 2018 21:34:57 
> Objet: [pve-devel] cpuflag: pcid needed in guest for good performance after meltdown 
> 
> 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 
> _______________________________________________ 
> pve-devel mailing list 
> pve-devel at pve.proxmox.com 
> https://pve.proxmox.com/cgi-bin/mailman/listinfo/pve-devel 
> 
> _______________________________________________ 
> pve-devel mailing list 
> pve-devel at pve.proxmox.com 
> https://pve.proxmox.com/cgi-bin/mailman/listinfo/pve-devel 
>