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Re: [Qemu-ppc] [PATCH] ppcr: Avoid decrementer related kvm exits
From: |
David Gibson |
Subject: |
Re: [Qemu-ppc] [PATCH] ppcr: Avoid decrementer related kvm exits |
Date: |
Fri, 14 Oct 2011 17:36:31 +1100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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.
--
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