Bjorn Helgaas <helgaas@xxxxxxxxxx> writes: > [+cc Rafael for question about ACPI method for PCI host bridge reset] > > On Sat, Nov 14, 2020 at 09:58:08PM +0100, Thomas Gleixner wrote: >> On Sat, Nov 14 2020 at 14:39, Bjorn Helgaas wrote: >> > On Sat, Nov 14, 2020 at 12:40:10AM +0100, Thomas Gleixner wrote: >> >> On Sat, Nov 14 2020 at 00:31, Thomas Gleixner wrote: >> >> > On Fri, Nov 13 2020 at 10:46, Bjorn Helgaas wrote: >> >> >> pci_device_shutdown() still clears the Bus Master Enable bit if we're >> >> >> doing a kexec and the device is in D0-D3hot, which should also disable >> >> >> MSI/MSI-X. Why doesn't this solve the problem? Is this because the >> >> >> device causing the storm was in PCI_UNKNOWN state? >> >> > >> >> > That's indeed a really good question. >> >> >> >> So we do that on kexec, but is that true when starting a kdump kernel >> >> from a kernel crash? I doubt it. >> > >> > Ah, right, I bet that's it, thanks. The kdump path is basically this: >> > >> > crash_kexec >> > machine_kexec >> > >> > while the usual kexec path is: >> > >> > kernel_kexec >> > kernel_restart_prepare >> > device_shutdown >> > while (!list_empty(&devices_kset->list)) >> > dev->bus->shutdown >> > pci_device_shutdown # pci_bus_type.shutdown >> > machine_kexec >> > >> > So maybe we need to explore doing some or all of device_shutdown() in >> > the crash_kexec() path as well as in the kernel_kexec() path. >> >> The problem is that if the machine crashed anything you try to attempt >> before starting the crash kernel is reducing the chance that the crash >> kernel actually starts. > > Right. > >> Is there something at the root bridge level which allows to tell the >> underlying busses to shut up, reset or go into a defined state? That >> might avoid chasing lists which might be already unreliable. > > Maybe we need some kind of crash_device_shutdown() that does the > minimal thing to protect the kdump kernel from devices. The kdump kernel does not use any memory the original kernel uses. Which should be a minimal and fairly robust level of protection until the device drivers can be loaded and get ahold of things. > The programming model for conventional PCI host bridges and PCIe Root > Complexes is device-specific since they're outside the PCI domain. > There probably *are* ways to do those things, but you would need a > native host bridge driver or something like an ACPI method. I'm not > aware of an ACPI way to do this, but I added Rafael in case he is. > > A crash_device_shutdown() could do something at the host bridge level > if that's possible, or reset/disable bus mastering/disable MSI/etc on > individual PCI devices if necessary. Unless I am confused DMA'ing to memory that is not already in use is completely broken wether or not you are using the kdump kernel. Eric
