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

Tue Jan 9 14:10:27 CET 2018

Am 09.01.2018 um 13:02 schrieb Alexandre DERUMIER:
>>> You mean: 
>>> -cpu qemu64,+pcid
> 
> yes.
> 
> (don't known the perf difference between qemu64 (without pcid), and intel cpu model (without pcid too) ? )

The results are nearly the same.

Same VM as before now model qemu64,+pcid:
real    0m13.870s
user    0m7.128s
sys     0m6.697s

qemu64:
real 0m25.214s
user 0m16.923s
sys 0m8.956s

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 12:57:30
> Objet: Re: [pve-devel] cpuflag: pcid needed in guest for good performance after meltdown
> 
> Am 09.01.2018 um 12:55 schrieb Alexandre DERUMIER: 
>>>> Yes - see an example which does a lot of syscalls: 
>>
>> and for qemu64 ? (is it possible to sent +pcid too ?) 
> 
> You mean: 
> -cpu qemu64,+pcid 
> ? 
> 
> 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 12:20:04 
>> Objet: Re: [pve-devel] cpuflag: pcid needed in guest for good performance after meltdown 
>>
>> 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 
>>>>>
>>>>
>>>
>>
> 