Re: Paravirt KVM capabilities

[Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>]

* 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, and 
has to switch it back to get back to guest user context. It might be
pretty fast with tagged TLBs, but not zero-cost.

	Ingo

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