[PATCH 1/1] pcie: Add hotplug detect state register to w1cmask

[Leonardo Bras]

When trying to migrate a machine type pc-q35-6.0 or lower, with this
cmdline options:

-device 
driver=pcie-root-port,port=18,chassis=19,id=pcie-root-port18,bus=pcie.0,addr=0x12
 \
-device 
driver=nec-usb-xhci,p2=4,p3=4,id=nex-usb-xhci0,bus=pcie-root-port18,addr=0x12.0x1

the following bug happens after all ram pages were sent:

qemu-kvm: get_pci_config_device: Bad config data: i=0x6e read: 0 device: 40 
cmask: ff wmask: 0 w1cmask:19
qemu-kvm: Failed to load PCIDevice:config
qemu-kvm: Failed to load pcie-root-port:parent_obj.parent_obj.parent_obj
qemu-kvm: error while loading state for instance 0x0 of device 
'0000:00:12.0/pcie-root-port'
qemu-kvm: load of migration failed: Invalid argument

This happens on pc-q35-6.0 or lower because of:
{ "ICH9-LPC", ACPI_PM_PROP_ACPI_PCIHP_BRIDGE, "off" }

In this scenario, hotplug_handler_plug() calls pcie_cap_slot_plug_cb(),
which sets the bus dev->config byte 0x6e with bit PCI_EXP_SLTSTA_PDS to 
signal PCI hotplug for the guest. After a while the guest will deal with
this hotplug and qemu will clear the above bit.

Then, during migration, get_pci_config_device() will compare the
configs of both the freshly created device and the one that is being
received via migration, which will differ due to the PCI_EXP_SLTSTA_PDS bit
and cause the bug to reproduce.

To avoid this fake incompatibility, there are two fields in PCIDevice that
can help:

.wmask: Used to implement R/W bytes, and
.w1cmask: Used to implement RW1C(Write 1 to Clear) bytes

According to pcie_cap_slot_init() the slot status register
(PCI_EXP_SLTSTA), in which PCI_EXP_SLTSTA_PDS is a flag, seems to fall
under w1cmask field, with makes sense due to the way signaling the hotplug
works.

So, add PCI_EXP_SLTSTA_PDS bit to w1cmask, so the fake incompatibility on
get_pci_config_device() does not abort the migration.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2215819
Signed-off-by: Leonardo Bras <leobras@redhat.com>
---
 hw/pci/pcie.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/pci/pcie.c b/hw/pci/pcie.c
index b8c24cf45f..2def1765a5 100644
--- a/hw/pci/pcie.c
+++ b/hw/pci/pcie.c
@@ -657,7 +657,7 @@ void pcie_cap_slot_init(PCIDevice *dev, PCIESlot *s)
                                PCI_EXP_SLTCTL_EIC);
 
     pci_word_test_and_set_mask(dev->w1cmask + pos + PCI_EXP_SLTSTA,
-                               PCI_EXP_HP_EV_SUPPORTED);
+                               PCI_EXP_HP_EV_SUPPORTED | PCI_EXP_SLTSTA_PDS);
 
     dev->exp.hpev_notified = false;
 
-- 
2.41.0