Re: [Qemu-devel] [Qemu-ppc] [PATCH] ppcr: Avoid decrementer related kvm exits

[David Gibson <david@gibson.dropbear.id.au>]

On Fri, Oct 14, 2011 at 07:30:09AM +0200, Alexander Graf wrote:
> 
> On 14.10.2011, at 07:19, David Gibson wrote:
> 
> > In __cpu_ppc_store_decr(), we set up a regular timer used to trigger
> > decrementer interrupts.  This is necessary to implement the decrementer
> > properly under TCG, but is unnecessary under KVM (true for both Book3S-PR
> > and Book3S-HV KVM variants), because the kernel handles generating and
> > delivering decrementer exceptions.
> > 
> > Under kvm, in fact, the timer causes expensive and unnecessary exits from
> > kvm to qemu.  This patch, therefore, disables setting the timer when kvm
> > is in use.
> > 
> > Signed-off-by: Anton Blanchard <anton@au1.ibm.com>
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> > hw/ppc.c |   25 ++++++++++++++-----------
> > 1 files changed, 14 insertions(+), 11 deletions(-)
> > 
> > diff --git a/hw/ppc.c b/hw/ppc.c
> > index 25b59dd..87aa4e5 100644
> > --- a/hw/ppc.c
> > +++ b/hw/ppc.c
> > @@ -658,21 +658,24 @@ static void __cpu_ppc_store_decr (CPUState *env, uint64_t *nextp,
> 
> Do we ever call store_decr in the kvm case? Isn't that only called
> from emulated mtdec?

Yes, from cpu_ppc_set_tb_clk().  Anton observed the kvm exits in the
wild, they're not theoretical.

Agh, which reminds me, I forgot to fixup the git author again.  The
patch should show authorship by Anton Blanchard <anton@au1.ibm.com>,
as in the s-o-b.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson