Re: Paravirt KVM capabilities

[Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>]

On Wed, 2007-01-10 at 11:52 +0100, Ingo Molnar wrote:
> * Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:
> 
> > If you have a CONFIG_PARAVIRT guest, I believe it will always be 
> > faster to run it without hardware assisted virtualization:
> > 
> > - you cannot eliminate vmexits due to host interrupts
> > - a hypercall will (probably) keep being more expensive than a syscall; 
> > it simply has a lot more work to do
> > - cr3 switches for CONFIG_PARAVIRT syscalls (which are necessary on 
> > x86_64) will probably become very cheap with tagged tlbs
> 
> but irq overhead is nothing in importance compared to basic syscall 
> overhead. KVM/HVM already runs guest kernel syscalls at native speed. 
> KVM/LL (or Xen) has to switch cr3s to enter guest kernel context

Err no, this isn't true.  See Documentation/lhype.txt or various blog
entries on the subject 8)  Both Xen and lhype get native syscall speeds
(within measurement error).

Rusty.



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