Re: KVM call minutes for Sept 21

[Gleb Natapov <gleb@redhat.com>]

On Mon, Sep 27, 2010 at 10:36:57AM +0200, Avi Kivity wrote:
>  On 09/26/2010 10:25 PM, Joerg Roedel wrote:
> >>  btw, speaking of drastic changes to nsvm, one thing I'd like to see is
> >>  the replacement of those kmaps with something like put_user_try() and
> >>  put_user_catch().  It should be as fast (or faster) than kmaps, and not
> >>  affect preemptibility.
> >
> >Yes, I want to get rid of them too. I thought about using
> >copy_from/to_user in the vmrun/vmexit path. I need to measure if this
> >has any performance impact, though.
> 
> copy_to_user() is slow since it is very generic.  put_user()
> generally translates to one instruction (perhaps a range check as
> well).  We can avoid the range check if we do it once for the entire
> vmcb page.
> 
> Gleb had something along those lines, it's useful for kvmclock as well.
> 
Well, since you asked to make it generic it uses copy_to_user() now :)
It tracks slot version so gfn_to_hva() translation is omitted most of
the times.

--
			Gleb.