Qemu Guest kernel 4.20-rc1 PCIe hotplug issue

[Shameerali Kolothum Thodi]

Hi Mika,

Since the commit commit 720d6a671a6e("PCI: pciehp: Do not handle events
if interrupts are masked"), the hotplug support on Qemu Guest(4.120-rc1)
with a vfio passthrough device seems to be broken. This is on an ARM64 platform.

I am booting a Guest with below command line options with the intention of
hot add a ixgbevf dev later,

./qemu-system-aarch64 -machine virt,kernel_irqchip=on,gic-version=3 -cpu host \
 -kernel Image_4.20-rc1 \
 -initrd rootfs-iperf.cpio \
 -device ioh3420,id=rp1 \
 -net none \
 -m 4096 \
 -nographic -D -d -enable-kvm \
 -append "console=ttyAMA0 root=/dev/vda -m 4096 rw pciehp.pciehp_debug=1
  pcie_ports=native searlycon=pl011,0x9000000"

But receives this on boot,

[    1.327852] pciehp 0000:00:01.0:pcie004: Timeout 
on hotplug command 0x03f1 (issued 1016 msec ago)
[    1.335842] pciehp 0000:00:01.0:pcie004: Timeout on hotplug command
0x03f1 (issued 1024 msec ago)
[    3.847843] pciehp 0000:00:01.0:pcie004: Failed to check link status
[    3.855843] pciehp 0000:00:01.0:pcie004: Timeout on hotplug command
0x02f1 (issued 2520 msec ago)
[    4.879846] pciehp 0000:00:01.0:pcie004: Timeout on hotplug command
0x06f1 (issued 1024 msec ago)
[    5.911840] pciehp 0000:00:01.0:pcie004: Timeout on hotplug command
0x06f1 (issued 2056 msec ago)
[    6.927844] pciehp 0000:00:01.0:pcie004: Timeout on hotplug command
0x07f1 (issued 1016 msec ago)
[    7.951843] pciehp 0000:00:01.0:pcie004: Timeout on hotplug command
0x0771 (issued 1024 msec ago)

Trying to hot add using "device_addvfio-pci,host=0000:01:10.1,id=net0,bus=rp1"
doesn't work either. And if I boot the guest with an assigned device
(-device vfio-pci,host=0000:01:10.1,id=net0,bus=rp1), I can see the dev listed in
the Guest but then hot remove doesn't work.

This all works on 4.19 and bisect points to the above mentioned commit, where an
additional check is added in pciehp_isr(),

-	 * Interrupts only occur in D3hot or shallower (PCIe r4.0, sec 6.7.3.4).
+	 * Interrupts only occur in D3hot or shallower and only if enabled
+	 * in the Slot Control register (PCIe r4.0, sec 6.7.3.4).
 	 */
-	if (pdev->current_state == PCI_D3cold)
+	if (pdev->current_state == PCI_D3cold ||
+	    (!(ctrl->slot_ctrl & PCI_EXP_SLTCTL_HPIE) && !pciehp_poll_mode))
 		return IRQ_NONE;

I think this doesn't work for the first time, where the cmd with PCI_EXP_SLTCTL_HPIE bit set
is written,
pciehp_probe()
  pcie_init_notification()
    pcie_enable_notification()
       pcie_do_write_cmd()

to begin with, ctrl->slot_ctrl = 0 in pciehp_isr() as this is only set once the write
is returned.

Or else I am missing something here. Please take a look and let me know.

Thanks,
Shameer