> -----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) Above values are not consistent but always in a delta of ~+-.002%. 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
