Re: Paravirt KVM capabilities

Rusty Russell
2007-01-11T00:42:00Z

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.