> -----Original Message----- > From: Liu Yu-B13201 > Sent: Thursday, October 20, 2011 12:35 PM > To: Bhushan Bharat-R65777; agraf@xxxxxxx > Cc: kvm-ppc@xxxxxxxxxxxxxxx; bharatb.yadav@xxxxxxxxx > 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; agraf@xxxxxxx > > Cc: kvm-ppc@xxxxxxxxxxxxxxx; bharatb.yadav@xxxxxxxxx > > 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; agraf@xxxxxxx > > > Cc: kvm-ppc@xxxxxxxxxxxxxxx; bharatb.yadav@xxxxxxxxx; > > Bhushan Bharat- > > > R65777 > > > Subject: RE: [PATCH v2] Fix DEC truncation for greater than > > > 0xffff_ffff/1000 > > > > > > > > > > > > > -----Original Message----- > > > > From: kvm-ppc-owner@xxxxxxxxxxxxxxx > > > > [mailto:kvm-ppc-owner@xxxxxxxxxxxxxxx] On Behalf Of Bharat Bhushan > > > > Sent: Wednesday, October 19, 2011 12:16 PM > > > > To: agraf@xxxxxxx > > > > Cc: kvm-ppc@xxxxxxxxxxxxxxx; bharatb.yadav@xxxxxxxxx; 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.bhushan@xxxxxxxxxxxxx> > > > > --- > > > > 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 -- To unsubscribe from this list: send the line "unsubscribe kvm-ppc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
