On 20.10.2011, at 09:18, Bhushan Bharat-R65777 wrote: > > >> -----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. So Yu are you ok with this patch? If so, please ack. Alex -- 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
