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

[Michael S. Tsirkin]

On Thu, Jun 29, 2023 at 04:01:41PM -0400, Peter Xu wrote:
> On Thu, Jun 29, 2023 at 03:33:06PM -0400, Michael S. Tsirkin wrote:
> > On Thu, Jun 29, 2023 at 01:01:53PM -0400, Peter Xu wrote:
> > > Hi, Leo,
> > > 
> > > Thanks for figuring this out.  Let me copy a few more potential reviewers
> > > from commit 17858a1695 ("hw/acpi/ich9: Set ACPI PCI hot-plug as default on
> > > Q35").
> > > 
> > > On Thu, Jun 29, 2023 at 06:05:00AM -0300, Leonardo Bras wrote:
> > > > 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.
> > 
> > Presence Detect State – This bit indicates the presence of an
> > adapter in the slot, reflected by the logical “OR” of the Physical
> > Layer in-band presence detect mechanism and, if present, any
> > out-of-band presence detect mechanism defined for the slot’s
> > corresponding form factor. Note that the in-band presence
> > detect mechanism requires that power be applied to an adapter
> > for its presence to be detected. Consequently, form factors that
> > require a power controller for hot-plug must implement a
> > physical pin presence detect mechanism.
> > RO
> > Defined encodings are:
> > 0b Slot Empty
> > 1b Card Present in slot
> > This bit must be implemented on all Downstream Ports that
> > implement slots. For Downstream Ports not connected to slots
> > (where the Slot Implemented bit of the PCI Express Capabilities
> > register is 0b), this bit must be hardwired to 1b.
> > 
> > 
> > And this seems to match what QEMU is doing: it clears on unplug
> > not after guest deals with hotplug.
> > 
> > 
> > > > 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.
> > 
> > So bit is set on source.
> > But why is the bit cleared on destination? This is the part I don't get.
> 
> My understanding is that when ACPI_PM_PROP_ACPI_PCIHP_BRIDGE is off for the
> device, we just won't ever PCI_EXP_SLTSTA_PDS bit?

Why?


> > 
> > 
> > > > 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
> > > 
> > > Is there one more option to clear the bit in cmask?
> > > 
> > > IIUC w1cmask means the guest can now write to this bit, but afaiu from the
> > > pcie spec it's RO.
> > 
> > Yes this bit must be RO.
> > 
> > > > 
> > > > 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>
> > > 
> > > Do we need a Fixes: and also the need to copy stable?
> > > 
> > > > ---
> > > >  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
> > > > 
> > > 
> > > -- 
> > > Peter Xu
> > 
> 
> -- 
> Peter Xu