Re: KVM call minutes for Sept 21

[Avi Kivity <avi@redhat.com>]

  On 09/27/2010 04:18 PM, Gleb Natapov wrote:
> 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.

Sure, that's easy to optimize later on.

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