On Fri, Oct 14, 2016 at 08:21:27PM +0200, Paolo Bonzini wrote: > On some benchmarks (e.g. netperf with ioeventfd disabled), APICv > posted interrupts turn out to be slower than interrupt injection via > KVM_REQ_EVENT. > > This patch optimizes a bit the IRR update, avoiding expensive atomic > operations in the common case where PI.ON=0 at vmentry or the PIR vector > is mostly zero. This saves at least 20 cycles (1%) per vmexit, as > measured by kvm-unit-tests' inl_from_qemu test (20 runs): > > | enable_apicv=1 | enable_apicv=0 > | mean stdev | mean stdev > ----------|-----------------|------------------ > before | 5826 32.65 | 5765 47.09 > after | 5809 43.42 | 5777 77.02 > > Of course, any change in the right column is just placebo effect. :) > The savings are bigger if interrupts are frequent. > > Signed-off-by: Paolo Bonzini <[email protected]> > --- > arch/x86/kvm/lapic.c | 6 ++++-- > arch/x86/kvm/vmx.c | 9 ++++++++- > 2 files changed, 12 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c > index 23b99f305382..63a442aefc12 100644 > --- a/arch/x86/kvm/lapic.c > +++ b/arch/x86/kvm/lapic.c > @@ -342,9 +342,11 @@ void __kvm_apic_update_irr(u32 *pir, void *regs) > u32 i, pir_val; > > for (i = 0; i <= 7; i++) { > - pir_val = xchg(&pir[i], 0); > - if (pir_val) > + pir_val = READ_ONCE(pir[i]); > + if (pir_val) { > + pir_val = xchg(&pir[i], 0); > *((u32 *)(regs + APIC_IRR + i * 0x10)) |= pir_val; > + } > } > } > EXPORT_SYMBOL_GPL(__kvm_apic_update_irr);
gcc doesn't seem to unroll this loop and it's probably worth unrolling it The following seems to do the trick for me on upstream - I didn't benchmark it though. Is there a kvm unit test for interrupts? ---> kvm: unroll the loop in __kvm_apic_update_irr. This is hot data path in interrupt-rich workloads, worth unrolling. Signed-off-by: Michael S. Tsirkin <[email protected]> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index b62c852..0c3462c 100644 --- a/arch/x86/kvm/lapic.c +++ b/arch/x86/kvm/lapic.c @@ -337,7 +337,8 @@ static u8 count_vectors(void *bitmap) return count; } -void __kvm_apic_update_irr(u32 *pir, void *regs) +void __attribute__((optimize("unroll-loops"))) +__kvm_apic_update_irr(u32 *pir, void *regs) { u32 i, pir_val;
