> -----Original Message-----
> From: Liu Yu-B13201
> Sent: Wednesday, October 19, 2011 4:28 PM
> To: Bhushan Bharat-R65777; ag...@suse.de
> Cc: kvm-ppc@vger.kernel.org; bharatb.ya...@gmail.com; Bhushan Bharat-
> R65777
> Subject: RE: [PATCH v2] Fix DEC truncation for greater than
> 0xffff_ffff/1000
>
>
>
> > -----Original Message-----
> > From: kvm-ppc-ow...@vger.kernel.org
> > [mailto:kvm-ppc-ow...@vger.kernel.org] On Behalf Of Bharat Bhushan
> > Sent: Wednesday, October 19, 2011 12:16 PM
> > To: ag...@suse.de
> > Cc: kvm-ppc@vger.kernel.org; bharatb.ya...@gmail.com; Bhushan
> > Bharat-R65777
> > Subject: [PATCH v2] Fix DEC truncation for greater than
> > 0xffff_ffff/1000
> >
> > kvmppc_emulate_dec() uses dec_nsec of type unsigned long and does
> > below calculation:
> >
> > dec_nsec = vcpu->arch.dec;
> > dec_nsec *= 1000;
> > This will truncate if DEC value "vcpu->arch.dec" is greater than
> > 0xffff_ffff/1000.
> > For example : For tb_ticks_per_usec = 4a, we can not set decrementer
> > more than ~58ms.
> >
> > Signed-off-by: Bharat Bhushan <bharat.bhus...@freescale.com>
> > ---
> > arch/powerpc/kvm/emulate.c | 12 +++++++-----
> > 1 files changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/powerpc/kvm/emulate.c b/arch/powerpc/kvm/emulate.c
> > index 8af3bad..e7f3da4 100644
> > --- a/arch/powerpc/kvm/emulate.c
> > +++ b/arch/powerpc/kvm/emulate.c
> > @@ -84,6 +84,7 @@ static bool kvmppc_dec_enabled(struct kvm_vcpu
> > *vcpu) void kvmppc_emulate_dec(struct kvm_vcpu *vcpu) {
> > unsigned long dec_nsec;
> > + unsigned long long dec_time;
> >
> > pr_debug("mtDEC: %x\n", vcpu->arch.dec);
> > #ifdef CONFIG_PPC_BOOK3S
> > @@ -103,11 +104,12 @@ void kvmppc_emulate_dec(struct kvm_vcpu *vcpu)
> > * host ticks. */
> >
> > hrtimer_try_to_cancel(&vcpu->arch.dec_timer);
> > - dec_nsec = vcpu->arch.dec;
> > - dec_nsec *= 1000;
> > - dec_nsec /= tb_ticks_per_usec;
> > - hrtimer_start(&vcpu->arch.dec_timer,
> > ktime_set(0, dec_nsec),
> > - HRTIMER_MODE_REL);
> > + dec_time = vcpu->arch.dec;
> > + dec_time *= 1000;
> > + do_div(dec_time, tb_ticks_per_usec);
> > + dec_nsec = do_div(dec_time, NSEC_PER_SEC);
> > + hrtimer_start(&vcpu->arch.dec_timer,
> > + ktime_set(dec_time, dec_nsec),
> > HRTIMER_MODE_REL);
> > vcpu->arch.dec_jiffies = get_tb();
> > } else {
> > hrtimer_try_to_cancel(&vcpu->arch.dec_timer);
> > --
> > 1.7.0.4
> >
>
> How does this impact performance?
> 64bits multiplication and division looks slow.
>
I tried running below test as guest, with and without this patch and tried to
find latency added by this patch. Also I run this for a list of timeouts (1, 2
, 4, 8, 16, 32ms) one by one.
- get TB (say a).
- set decrementer in auto reload mode.
- wait for 1000 timebase interrupts.
- Get timebase delta (b = get_tb - a).
(b1 - b2) <=> b1 with this patch and b2 without this
patch. And roughly I found any impact. For example:
For 1ms = ( 48a19d8 - 48a1459) = 0x57f = .0018%
For 32ms = (90fdfa23 - 90fdfe79) = -(0x456)
Above values are not consistent but always in a delta of ~+-.002%.
Thanks
-Bharat
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