KVM: Kick resamplefd for split kernel irqchip

KVM: Kick resamplefd for split kernel irqchip

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>
Message-Id: <20200318145204.74483-5-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>