On Fri, Oct 14, 2011 at 08:44:06AM +0200, Alexander Graf wrote:
>
> On 14.10.2011, at 08:36, David Gibson wrote:
>
> > 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 <an...@au1.ibm.com>
> >>> Signed-off-by: David Gibson <da...@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 <an...@au1.ibm.com>,
> > as in the s-o-b.
>
> Wouldn't a simple
>
> if (kvm_enabled()) {
> return;
> }
>
> in the beginning of the function make more sense? There's no code
> connecting the in-qemu and the in-kvm decrementors atm, so any logic
> applying to the in-qemu one is moot for kvm.
Uh.. I guess so. I wasn't 100% sure the last bit of code in the
function wouldn't have some effect on kvm. But I guess it doesn't;
I'll revise.
--
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
