[Qemu-devel] how to pass pcid to guest?

[Qemu-devel] how to pass pcid to guest?

[Stefan Priebe - Profihost AG]

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

Qemu is 2.9.1

Thanks!

Greets,
Stefan

Re: [Qemu-devel] how to pass pcid to guest?

[Eric Blake]

[-- Attachment #1: Type: text/plain, Size: 624 bytes --]

On 01/08/2018 02:03 PM, Stefan Priebe - Profihost AG wrote:
> 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).

Indeed; you are still waiting on the qemu patch mentioned here:
https://www.qemu.org/2018/01/04/spectre/

which is still undergoing the review process, but should be up (in the
form of 2.11.1) "in the next few days".

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 619 bytes --]

Re: [Qemu-devel] how to pass pcid to guest?

[Stefan Priebe - Profihost AG]

Am 08.01.2018 um 23:07 schrieb Eric Blake:
> On 01/08/2018 02:03 PM, Stefan Priebe - Profihost AG wrote:
>> 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).
> 
> Indeed; you are still waiting on the qemu patch mentioned here:
> https://www.qemu.org/2018/01/04/spectre/
> 
> which is still undergoing the review process, but should be up (in the
> form of 2.11.1) "in the next few days".

OK thanks as the performance difference is significant:

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

Re: [Qemu-devel] how to pass pcid to guest?

[Daniel P. Berrange]

On Mon, Jan 08, 2018 at 04:07:53PM -0600, Eric Blake wrote:
> On 01/08/2018 02:03 PM, Stefan Priebe - Profihost AG wrote:
> > 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).
> 
> Indeed; you are still waiting on the qemu patch mentioned here:
> https://www.qemu.org/2018/01/04/spectre/
> 
> which is still undergoing the review process, but should be up (in the
> form of 2.11.1) "in the next few days".

There's two different things here. The Spectre patch you mention is about
exposing a new CPUID feature 'ibrs' that is added to fix Spectre. The 'pcid'
feature Stefan mentions is a pre-existing feature that is not a security fix,
but rather a way to improve the performance when the KAISER/KPTI patches are
enabled for Meltdown.

'pcid' is available in QEMU's  Haswell, Broadwell & Skylake based CPU
models by default. For other named CPU models, you can turn it on using
',+pcid' after the model name. eg  '-cpu  Westmere,+pcid' (as long as your
host supports it of course)

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|