Re: [PATCH v2 4/5] KVM: Kick resamplefd for split kernel irqchip

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 28 Feb 2020 11:15:02 -0500
Peter Xu <peterx@redhat.com> wrote:

> This is majorly only for X86 because that's the only one that supports
> split irqchip for now.
> 
> When the irqchip is split, we face a dilemma that KVM irqfd will be
> enabled, however the slow irqchip is still running in the userspace.
> It means that the resamplefd in the kernel irqfds won't take any
> effect and it will miss to ack INTx interrupts on EOIs.
> 
> One example is split irqchip with VFIO INTx, which will break if we
> use the VFIO INTx fast path.
> 
> This patch can potentially supports the VFIO fast path again for INTx,
> that the IRQ delivery will still use the fast path, while we don't
> need to trap MMIOs in QEMU for the device to emulate the EIOs (see the
> callers of vfio_eoi() hook).  However the EOI of the INTx will still
> need to be done from the userspace by caching all the resamplefds in
> QEMU and kick properly for IOAPIC EOI broadcast.
> 
> This is tricky because in this case the userspace ioapic irr &
> remote-irr will be bypassed.  However such a change will greatly boost
> performance for assigned devices using INTx irqs (TCP_RR boosts 46%
> after this patch applied).
> 
> When the userspace is responsible for the resamplefd kickup, don't
> register it on the kvm_irqfd anymore, because on newer kernels (after
> commit 654f1f13ea56, 5.2+) the KVM_IRQFD will fail if with both split
> irqchip and resamplefd.  This will make sure that the fast path will
> work for all supported kernels.
> 
> https://patchwork.kernel.org/patch/10738541/#22609933
> 
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  accel/kvm/kvm-all.c    | 85 +++++++++++++++++++++++++++++++++++++++++-
>  accel/kvm/trace-events |  1 +
>  hw/intc/ioapic.c       | 23 +++++++++++-
>  include/sysemu/kvm.h   |  7 ++++
>  4 files changed, 112 insertions(+), 4 deletions(-)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index d49b74512a..89771ea114 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -159,9 +159,65 @@ static const KVMCapabilityInfo kvm_required_capabilites[] = {
>  static NotifierList kvm_irqchip_change_notifiers =
>      NOTIFIER_LIST_INITIALIZER(kvm_irqchip_change_notifiers);
>  
> +struct KVMResampleFd {
> +    int gsi;
> +    EventNotifier *resample_event;
> +    QLIST_ENTRY(KVMResampleFd) node;
> +};
> +typedef struct KVMResampleFd KVMResampleFd;
> +
> +/*
> + * Only used with split irqchip where we need to do the resample fd
> + * kick for the kernel from userspace.
> + */
> +static QLIST_HEAD(, KVMResampleFd) kvm_resample_fd_list =
> +    QLIST_HEAD_INITIALIZER(kvm_resample_fd_list);
> +
>  #define kvm_slots_lock(kml)      qemu_mutex_lock(&(kml)->slots_lock)
>  #define kvm_slots_unlock(kml)    qemu_mutex_unlock(&(kml)->slots_lock)
>  
> +static inline void kvm_resample_fd_remove(int gsi)
> +{
> +    KVMResampleFd *rfd;
> +
> +    QLIST_FOREACH(rfd, &kvm_resample_fd_list, node) {
> +        if (rfd->gsi == gsi) {
> +            QLIST_REMOVE(rfd, node);
> +            g_free(rfd);
> +            break;
> +        }
> +    }
> +}
> +
> +static inline void kvm_resample_fd_insert(int gsi, EventNotifier *event)
> +{
> +    KVMResampleFd *rfd = g_new0(KVMResampleFd, 1);
> +
> +    rfd->gsi = gsi;
> +    rfd->resample_event = event;
> +
> +    QLIST_INSERT_HEAD(&kvm_resample_fd_list, rfd, node);
> +}
> +
> +bool kvm_resample_fd_notify(int gsi)
> +{
> +    KVMResampleFd *rfd;
> +
> +    if (!kvm_irqchip_is_split()) {
> +        return false;
> +    }
> +
> +    QLIST_FOREACH(rfd, &kvm_resample_fd_list, node) {
> +        if (rfd->gsi == gsi) {
> +            event_notifier_set(rfd->resample_event);
> +            trace_kvm_resample_fd_notify(gsi);
> +            return true;
> +        }
> +    }
> +
> +    return false;
> +}
> +
>  int kvm_get_max_memslots(void)
>  {
>      KVMState *s = KVM_STATE(current_accel());
[snip]  
> diff --git a/hw/intc/ioapic.c b/hw/intc/ioapic.c
> index 15747fe2c2..13921b333d 100644
> --- a/hw/intc/ioapic.c
> +++ b/hw/intc/ioapic.c
> @@ -236,8 +236,27 @@ void ioapic_eoi_broadcast(int vector)
>          for (n = 0; n < IOAPIC_NUM_PINS; n++) {
>              entry = s->ioredtbl[n];
>  
> -            if ((entry & IOAPIC_VECTOR_MASK) != vector ||
> -                ((entry >> IOAPIC_LVT_TRIGGER_MODE_SHIFT) & 1) != IOAPIC_TRIGGER_LEVEL) {
> +            if ((entry & IOAPIC_VECTOR_MASK) != vector) {
> +                continue;
> +            }
> +
> +            /*
> +             * When IOAPIC is in the userspace while APIC is still in
> +             * the kernel (i.e., split irqchip), we have a trick to
> +             * kick the resamplefd logic for registered irqfds from
> +             * userspace to deactivate the IRQ.  When that happens, it
> +             * means the irq bypassed userspace IOAPIC (so the irr and
> +             * remote-irr of the table entry should be bypassed too
> +             * even if interrupt come), then we don't need to clear
> +             * the remote-IRR and check irr again because they'll
> +             * always be zeros.
> +             */
> +            if (kvm_resample_fd_notify(n)) {
> +                continue;
> +            }

It seems the problem I reported is here.  In my configuration virtio-blk
and an assigned e1000e share an interrupt.  virtio-blk is initializing
and apparently triggers an interrupt.  The vfio-pci device is
configured for INTx though not active yet, but kvm_resample_fd_notify()
kicks the fd here, so we continue.  If I remove the continue here both
devices seem to work, but I don't claim to understand the condition
we're trying to continue for here yet.  This series needs more testing
with shared interrupts.  Thanks,

Alex