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

[Stefan Priebe - Profihost AG]

Am 09.01.2018 um 10:43 schrieb Alexandre DERUMIER:
>>> That's bad as pcid is very important to performance for meltdown fixes
>>> in the linux kernel.
> 
> I wonder the difference of performance for
> 
> - qemu64|kvm64 cpu model
> - intel cpu model
> - intel + pcid cpu model ?
> 
> (currently I'm running mainly qemu64 because I don't need advanced cpu flag)
> 
> Do you have already done some benchmarks ?


Yes - see an example which does a lot of syscalls:

no tasks running other than du:

no pcid:
# time for i in $(seq 1 1 50); do du -sx /; done
...
real    0m26.614s
user    0m17.548s
sys     0m9.056s


kvm started with +pcid:
# time for i in $(seq 1 1 50); do du -sx /; done
...
real    0m14.734s
user    0m7.755s
sys     0m6.973s

Greets,
Stefan

> 
> ----- Mail original -----
> De: "Stefan Priebe, Profihost AG" <s.priebe at profihost.ag>
> À: "aderumier" <aderumier at odiso.com>
> Cc: "pve-devel" <pve-devel at pve.proxmox.com>
> Envoyé: Mardi 9 Janvier 2018 10:20:39
> Objet: Re: [pve-devel] cpuflag: pcid needed in guest for good performance after meltdown
> 
> 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 
>>> _______________________________________________ 
>>> 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 
>>>
>>
>