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

[Alexandre DERUMIER]

>>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.

how does it solve the problem, if you update at start ? (if you take the current qemu version, and update the conf file).


we should need something external to qemu, like proxmox_config_version or something like that.
But I'm not sure it's easy to maintain. (maybe some user want to keep an old qemu version for example, with last qemu-server code)

As alternative, we could patch qemu to have something like version : 2.11.x.y  (where x is the minor version from qemu, and y minor version for proxmox)



----- 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 
>