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

[David Gibson]

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 <address@hidden>
> >>> Signed-off-by: David Gibson <address@hidden>
> >>> ---
> >>> 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 <address@hidden>,
> > 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