[libvirt] [PATCH] "pclmuldq" was introduced with Westmere, not Sandy Bridge. This feature is important to get proper performance for aes-128-gcm in openssl, an important cipher for https communication.

[Itamar Heim]

On 10/14/2014 01:28 PM, Jan-Frode Myklebust wrote:
> On Wed, Oct 08, 2014 at 09:49:35AM +0200, Jiri Denemark wrote:
>>
>> Technically you are correct and even QEMU added this feature to Westmere
>> in April 2013. However, our goal is to provide stable virtual hardware
>> that doesn't change when, e.g., a domain is migrated to another machine
>> (let's ignore the fact we don't currently enforce such stability for CPU
>> models/features because of missing functionality in both QEMU and
>> libvirt). Thus we should not really change existing CPU models. We may
>> be able to do that in the future depending how (if ever) we solve the
>> CPU definition probing in QEMU and how libvirt will make use of it to
>> really enforce stable ABI for guest operating systems.
>
>
> Right, I see the problem, but am having a bit trouble accepting that all
> our 20 RHEV-H westmere hypervisors are basicly downgraded to nehalem
> feature-set permanently because if this, and we probably have to live
> with these servers for quite some time.  If you can't fix existing virtual
> cpu types, maybe you should add a "westmere-full-feature" cpu type, or
> similar? And probably also add "rdtscp" which also is missing from the
> virtual westmere.

if libvirt would change the definition, then a mixed cluster (old and 
new libvirt) would be broken. it will only work if its a new cpu model

>
>
>>
>> Moreover, it's trivial to enable the feature in domain XML:
>>
>
> We're using RHEV, and RHEV-H on all hypervisors, so not so easy to fix
> for us.. Have opened ticket with Red Hat for this.

you can edit the RHEV config at per OS level or cluster level to edit 
the cpu (not sure the global change this will persist after an upgrade).

>
>
>
>
>    -jf
>
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