Re: [PATCH v3 2/6] KVM: X86: Implement PV IPIs in linux guest

[Wanpeng Li]

On Fri, 20 Jul 2018 at 00:28, Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx> wrote:
>
> 2018-07-03 14:21+0800, Wanpeng Li:
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> > Implement paravirtual apic hooks to enable PV IPIs.
> >
> > apic->send_IPI_mask
> > apic->send_IPI_mask_allbutself
> > apic->send_IPI_allbutself
> > apic->send_IPI_all
> >
> > The PV IPIs supports maximal 128 vCPUs VM, it is big enough for cloud
> > environment currently, supporting more vCPUs needs to introduce more
> > complex logic, in the future this might be extended if needed.
> >
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> > Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> > ---
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > @@ -454,6 +454,71 @@ static void __init sev_map_percpu_data(void)
> >  }
> >
> >  #ifdef CONFIG_SMP
> > +
> > +#ifdef CONFIG_X86_64
> > +static void __send_ipi_mask(const struct cpumask *mask, int vector)
> > +{
> > +     unsigned long flags, ipi_bitmap_low = 0, ipi_bitmap_high = 0;
> > +     int cpu, apic_id;
> > +
> > +     if (cpumask_empty(mask))
> > +             return;
> > +
> > +     local_irq_save(flags);
> > +
> > +     for_each_cpu(cpu, mask) {
> > +             apic_id = per_cpu(x86_cpu_to_apicid, cpu);
> > +             if (apic_id < BITS_PER_LONG)
> > +                     __set_bit(apic_id, &ipi_bitmap_low);
> > +             else if (apic_id < 2 * BITS_PER_LONG)
> > +                     __set_bit(apic_id - BITS_PER_LONG, &ipi_bitmap_high);
>
> It'd be nicer with 'unsigned long ipi_bitmap[2]' and a single
>
>         __set_bit(apic_id, ipi_bitmap);
>
> > +     }
> > +
> > +     kvm_hypercall3(KVM_HC_SEND_IPI, ipi_bitmap_low, ipi_bitmap_high, vector);
>
> and
>
>         kvm_hypercall3(KVM_HC_SEND_IPI, ipi_bitmap[0], ipi_bitmap[1], vector);
>
> Still, the main problem is that we can only address 128 APICs.
>
> A simple improvement would reuse the vector field (as we need only 8
> bits) and put a 'offset' in the rest.  The offset would say which
> cluster of 128 are we addressing.  24 bits of offset results in 2^31
> total addressable CPUs (we probably should even use that many bits).
> The downside of this is that we can only address 128 at a time.
>
> It's basically the same as x2apic cluster mode, only with 128 cluster
> size instead of 16, so the code should be a straightforward port.
> And because x2apic code doesn't seem to use any division by the cluster
> size, we could even try to use kvm_hypercall4, add ipi_bitmap[2], and
> make the cluster size 192. :)
>
> But because it is very similar to x2apic, I'd really need some real
> performance data to see if this benefits a real workload.

Thanks for your review, Radim! :) I will find another real benchmark
instead of the micro one to evaluate the performance.

> Hardware could further optimize LAPIC (apicv, vapic) in the future,
> which we'd lose by using paravirt.
>
> e.g. AMD's acceleration should be superior to this when using < 8 VCPUs
> as they can use logical xAPIC and send without VM exits (when all VCPUs
> are running).
>
> > +
> > +     local_irq_restore(flags);
> > +}
> > +
> > +static void kvm_send_ipi_mask(const struct cpumask *mask, int vector)
> > +{
> > +     __send_ipi_mask(mask, vector);
> > +}
> > +
> > +static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int vector)
> > +{
> > +     unsigned int this_cpu = smp_processor_id();
> > +     struct cpumask new_mask;
> > +     const struct cpumask *local_mask;
> > +
> > +     cpumask_copy(&new_mask, mask);
> > +     cpumask_clear_cpu(this_cpu, &new_mask);
> > +     local_mask = &new_mask;
> > +     __send_ipi_mask(local_mask, vector);
> > +}
> > +
> > +static void kvm_send_ipi_allbutself(int vector)
> > +{
> > +     kvm_send_ipi_mask_allbutself(cpu_online_mask, vector);
> > +}
> > +
> > +static void kvm_send_ipi_all(int vector)
> > +{
> > +     __send_ipi_mask(cpu_online_mask, vector);
>
> These should be faster when using the native APIC shorthand -- is this
> the "Broadcast" in your tests?

Not true, .send_IPI_all almost no callers though linux apic drivers
implement this hook, in addition, shortcut is not used for x2apic
mode(__x2apic_send_IPI_dest()), and very limited using in other
scenarios according to linux apic drivers.

>
> > +}
> > +
> > +/*
> > + * Set the IPI entry points
> > + */
> > +static void kvm_setup_pv_ipi(void)
> > +{
> > +     apic->send_IPI_mask = kvm_send_ipi_mask;
> > +     apic->send_IPI_mask_allbutself = kvm_send_ipi_mask_allbutself;
> > +     apic->send_IPI_allbutself = kvm_send_ipi_allbutself;
> > +     apic->send_IPI_all = kvm_send_ipi_all;
> > +     pr_info("KVM setup pv IPIs\n");
> > +}
> > +#endif
> > +
> >  static void __init kvm_smp_prepare_cpus(unsigned int max_cpus)
> >  {
> >       native_smp_prepare_cpus(max_cpus);
> > @@ -626,6 +691,11 @@ static uint32_t __init kvm_detect(void)
> >
> >  static void __init kvm_apic_init(void)
> >  {
> > +#if defined(CONFIG_SMP) && defined(CONFIG_X86_64)
> > +     if (kvm_para_has_feature(KVM_FEATURE_PV_SEND_IPI) &&
> > +             num_possible_cpus() <= 2 * BITS_PER_LONG)
>
> It looks that num_possible_cpus() is actually NR_CPUS, so the feature
> would never be used on a standard Linux distro.
> And we're using APIC_ID, which can be higher even if maximum CPU the
> number is lower.  Just remove it.

Will do.

Regards,
Wanpeng Li