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[PATCH v3 0/5] vfio/pci: Fix up breakage against split irqchip and INTx

From: Peter Xu
Subject: [PATCH v3 0/5] vfio/pci: Fix up breakage against split irqchip and INTx
Date: Tue, 17 Mar 2020 15:50:37 -0400

I believe I missed 5.0... Anyway still good to post it before being

- collect r-bs for Eric
- unconditionally call kvm_resample_fd_notify(), change comment [Alex]
- remove the split irqchip check in kvm_resample_fd_notify(), then let
  it return nothing [Alex]
- test against shared irq to make sure it won't break

- pick tags
- don't register resamplefd with KVM kernel when the userspace
  resamplefd path is enabled (should enable fast path on new kernels)
- fix resamplefd mem leak
- fix commit message of patch 4 [Eric]
- let kvm_resample_fd_notify() return a boolean, skip ioapic check if
  returned true
- more comments here and there in the code to state the fact that
  userspace ioapic irr & remote-irr are bypassed [Paolo]

VFIO INTx is not working with split irqchip.  On new kernels KVM_IRQFD
will directly fail with resamplefd attached so QEMU will automatically
fallback to the INTx slow path.  However on old kernels it's still

Only until recently I noticed that this could also break PXE boot for
assigned NICs [1].  My wild guess is that the PXE ROM will be mostly
using INTx as well, which means we can't bypass that even if we
enables MSI for the guest kernel.

This series tries to first fix this issue function-wise, then speed up
for the INTx again with resamplefd (mostly following the ideas
proposed by Paolo one year ago [2]).  My TCP_RR test shows that:

  - Before this series: this is broken, no number to show

  - After patch 1 (enable slow path): get 63% perf comparing to full
    kernel irqchip

  - After whole series (enable fast path partly, irq injection will be
    the same as fast path, however userspace needs to intercept for
    EOI broadcast to resamplefd, though should still be faster than
    the MMIO trick for intx eoi): get 93% perf comparing to full
    kernel irqchip, which is a 46% performance boost

I think we can consider to apply patch 1 even sooner than the rest of
the series to unbreak intx+split first.

The whole test matrix for reference:

  | IRQ type | irqchip | TCP_STREAM (Gbps) | TCP_RR (pps) | note               |
  | msi      | on      |              9.39 |        17567 |                    |
  | nomsi    | on      |              9.29 |        14056 |                    |
  | msi      | split   |              9.36 |        17330 |                    |
  | nomsi    | split   |                 / |            / | currently broken   |
  | nomsi    | split   |              8.98 |         8977 | after patch 1      |
  | nomsi    | split   |              9.21 |        13142 | after whole series |

Any review comment is welcomed.  Thanks,

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1786404
[2] https://patchwork.kernel.org/patch/10738541/#22609933

Peter Xu (5):
  vfio/pci: Disable INTx fast path if using split irqchip
  vfio/pci: Use kvm_irqchip_add_irqfd_notifier_gsi() for irqfds
  KVM: Pass EventNotifier into kvm_irqchip_assign_irqfd
  KVM: Kick resamplefd for split kernel irqchip
  Revert "vfio/pci: Disable INTx fast path if using split irqchip"

 accel/kvm/kvm-all.c    | 95 ++++++++++++++++++++++++++++++++++++++----
 accel/kvm/trace-events |  1 +
 hw/intc/ioapic.c       | 25 ++++++++++-
 hw/vfio/pci.c          | 37 +++++++---------
 include/sysemu/kvm.h   |  4 ++
 5 files changed, 130 insertions(+), 32 deletions(-)


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