Re: [RFC PATCH] PCI, kdump: Clear bus master bit upon shutdown in kdump kernel

[Kairui Song]

On Sat, Jan 11, 2020 at 8:45 AM Baoquan He <bhe@xxxxxxxxxx> wrote:
> On 01/10/20 at 04:00pm, Jerry Hoemann wrote:
> > > I am not understanding this failure mode either. That code in
> > > pci_device_shutdown() was added originally to address this very issue.
> > > The patch 4fc9bbf98fd6 ("PCI: Disable Bus Master only on kexec reboot")
> > > shut down any errant DMAs from PCI devices as we kexec a new kernel. In
> > > this new patch, this is the same code path that will be taken again when
> > > kdump kernel is shutting down. If the errant DMA problem was not fixed
> > > by clearing Bus Master bit in this path when kdump kernel was being
> > > kexec'd, why does the same code path work the second time around when
> > > kdump kernel is shutting down? Is there more going on that we don't
> > > understand?
> > >
> >
> >   Khalid,
> >
> >   I don't believe we execute that code path in the crash case.
> >
> >   The variable kexec_in_progress is set true in kernel_kexec() before calling
> >   machine_kexec().  This is the fast reboot case.
> >
> >   I don't see kexec_in_progress set true elsewhere.
> >
> >
> >   The code path for crash is different.
> >
> >   For instance, panic() will call
> >       -> __crash_kexec()  which calls
> >               -> machine_kexec().
> >
> >  So the setting of kexec_in_progress is bypassed.
>
> Yeah, it's a differet behaviour than kexec case. I talked to Kairui, the
> patch log may be not very clear. Below is summary I got from my
> understanding about this issue:
>
> ~~~~~~~~~~~~~~~~~~~~~~~
> Problem:
>
> When crash is triggered, system jumps into kdump kernel to collect
> vmcore and dump out. After dumping is finished, kdump kernel will try
> ty reboot to normal kernel. This hang happened during kdump kernel
> rebooting, when dumping is network dumping, e.g ssh/nfs, local storage
> is HPSA.
>
> Root cause:
>
> When configuring network dumping, only network driver modules are added
> into kdump initramfs. However, the storage HPSA pcie device is enabled
> in 1st kernel, its status is PCI_D3hot. When crashed system jumps to kdump
> kernel, we didn't shutdown any device for safety and efficiency. Then
> during kdump kernel boot up, the pci scan will get hpsa device and only
> initialize its status as pci_dev->current_state = PCI_UNKNOWN. This
> pci_dev->current_state will be manipulated by the relevant device
> driver. So HPSA device will never have chance to calibrate its status,
> and can't be shut down by pci_device_shutdown() called by reboot
> service. It's still PCI_D3hot, then crash happened when system try to
> shutdown its upper bridge.
>
> Fix:
>
> Here, Kairui uses a quirk to get PM state and mask off value bigger than
> PCI_D3cold. Means, all devices will get PM state
> pci_dev->current_state = PCI_D0 or PCI_D3hot

Or to put it simple, I just synced the actual PM state into
pci_dev->current_state using a quirk, for kdump kernel only.

> Finally, during kdump
> reboot stage, this device can be shut down successfully by clearing its
> master bit.
>
> ~~~~~~~~~~~~~~~
>
> About this patch, I think the quirk getting active PM state for all devices
> may be risky, it will impact normal kernel too which doesn't have this issue.
>
> Wondering if there's any other way to fix or work around it.
>

Thank you for the detailed description!

-- 
Best Regards,
Kairui Song