RE: [PATCH] PCI: hv: Fix a crash in hv_pci_restore_msi_msg() during hibernation

[Dexuan Cui]

> From: Michael Kelley (LINUX) <mikelley@xxxxxxxxxxxxx>
> Sent: Tuesday, August 15, 2023 5:35 PM
> > [...]
> > For a Linux VM with a NVIDIA GPU running on Hyper-V, before the GPU
> driver
> > is installed, hibernating the VM will trigger a panic: if the GPU driver
> > is not installed and loaded, MSI-X/MSI is not enabled on the device, so
> > pdev->dev.msi.data is NULL, and msi_lock_descs(&pdev->dev) causes the
> > NULL pointer dereference. Fix this by checking pdev->dev.msi.data.
> 
> Is the scenario here a little broader than just the NVIDIA GPU driver?  For
> any virtual PCI device that is presented in the guest VM as a VMBus device,
> the driver might not be installed.  There could have been some initial
> problem getting the driver installed, or it might have been manually
> uninstalled later in the life of the VM.  Also the host might have rescinded
> the virtual PCI device and added it back later, creating another opportunity
> where the driver might not be loaded.  In any case, it seems like we could
> have the VMBus aspects of the device setup, but not the driver for the
> device.  This suspend/resume code in pci-hyperv.c is all about handling
> the VMBus aspects of the device anyway.

Good point! The bug also affects other PCI devices, e.g. if I unload mlx5_core
and let the VM with a Mellanox VF NIC hibernate, I hit the same NULL
pointer dereference.

> Assuming my thinking is correct, is there some Hyper-V/VMBus setting
> owned by the pci-hyperv.c driver that would be better to test here than
> the low-level dev.msi.data pointer?  The MSI code rework that added

IMO there is no easy and reliable way in Hyper-V/VMBus/pci-hyperv to 
tell if MSI/MSI-X is enabled for a PCI device. We can potentially track the
MSI/MSI-X irqs in hv_compose_msi_msg() and hv_irq_unmask(), but
IMO that's not very easy and may be inaccurate.

> the descriptor lock encapsulates the internals with appropriate accessor
> functions, and reaching in to directly test dev.msi.data violates that
> encapsulation.

I agree. 

Compared with:
	if (!pdev->dev.msi.data)
		return 0;

I think it's better to use this:
            if (!pdev->msi_enabled && !pdev->msix_enabled)
		return 0;

pdev-> msix_enabled has been used in many drivers, e.g.

"arch/x86/pci/xen.c": xen_pv_teardown_msi_irqs()
"drivers/hid/intel-ish-hid/ipc/pci-ish.c": ish_probe()
"drivers/infiniband/hw/vmw_pvrdma/pvrdma_main.c": pvrdma_intr0_handler()
"drivers/scsi/vmw_pvscsi.c": pvscsi_probe()
and more.

So it looks like pdev-> msix_enabled is a legit and stable API.
I'll post v2 with it. I'll update the changelog accordingly.
Please let me know if you have concerns about it.