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

From: Alexander Graf
Subject: Re: [Qemu-ppc] [PATCH] ppcr: Avoid decrementer related kvm exits
Date: Fri, 14 Oct 2011 08:44:06 +0200

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()) {

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.


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