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

[Stefan Priebe - Profihost AG]

Am 09.01.2018 um 09:18 schrieb 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."

That's bad as pcid is very important to performance for meltdown fixes
in the linux kernel.

Stefan

> 
> ----- 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 
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>