Re: What is CR3 caching?

["Pranav Peshwe" <pranavpeshwe@xxxxxxxxx>]

On Sat, Jul 5, 2008 at 8:40 PM, Mulyadi Santosa <mulyadi.santosa@xxxxxxxxx> wrote:

Guessing..

On Thu, Jul 3, 2008 at 11:19 PM, Peter Teoh <htmldeveloper@xxxxxxxxx> wrote:
> http://lkml.org/lkml/2007/1/5/213
>
> Question here is:   what is CR3 caching?  (few first line of email)?

probably it means it is stored in somekind of "fake" CR3 register. As
you know that storing/reading something to/from RAM is expensive
compared to accessing registers, so if you can "cache" them in
register, it would be damn faster...

Note, that's VT feature...meaning it is used for virtualization. That
makes sense, because vm exit/enter might happen so many times....and
that includes switching between many CR3 furiously....multiple times
more frequently than normal context switching between processes.

Hello,
         From Ingo Molnar's mail (which starts that thread), what i could infer is:
As Mulyadi states, CR3 caching is used during  virtualization. Normally, any change in CR3 would cause a switch out of the VMX mode into the hypervisor. But, Intel seems to have employed a 4 entried cache which can be made to store CR3 values for processes running in the guest. If, the changed CR3 value is one of the 4 in the 'CR3 target cache' then, an expensive VM exit to the hypervisor is avoided. 
This is how, i guess, the KVM trunk+paravirt/c3 performance is 'faster by a factor of 4' as compared to KVM trunk for 2 tasks and, decreases by a considerable amount for more than 4 tasks.

HTH. Kindly CMIIW.

Best regards,
Pranav