> -----Original Message-----
> From: Liu Yu-B13201
> Sent: Thursday, October 20, 2011 12:35 PM
> To: Bhushan Bharat-R65777; ag...@suse.de
> Cc: kvm-ppc@vger.kernel.org; bharatb.ya...@gmail.com
> Subject: RE: [PATCH v2] Fix DEC truncation for greater than
> 0xffff_ffff/1000
>
>
>
> > -----Original Message-----
> > From: Bhushan Bharat-R65777
> > Sent: Thursday, October 20, 2011 2:41 PM
> > To: Liu Yu-B13201; ag...@suse.de
> > Cc: kvm-ppc@vger.kernel.org; bharatb.ya...@gmail.com
> > Subject: RE: [PATCH v2] Fix DEC truncation for greater than
> > 0xffff_ffff/1000
> >
> >
> >
> > > -----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)
>
> Doesn't (b1 - b2) mean difference of the last one interrupt between have
> patch and havenot patch?
> The time of previous 999 interrupts is hidden in the cpu idle time.
>
Probably I have not described properly. b1 and b2 are delta, not timestamp. In
this case I run this test with patch
Print on console the total time (in TB tick) for which this test runs.
Which includes time of all 1000 interrupts.
Then I exit and rerun the above test case without patch
Then mannualy calculated difference/percentage etc.
Also if you see timebase delta, it suggest that it is not timebase difference
of one decrementer.
Thanks
-Bharat
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