Date:   Wed, 22 Jul 2020 10:21:23 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Kairui Song <kasong@...hat.com>
Cc:     Baoquan He <bhe@...hat.com>,
        Deepa Dinamani <deepa.kernel@...il.com>, jroedel@...e.de,
        Myron Stowe <myron.stowe@...hat.com>,
        linux-pci@...r.kernel.org, kexec@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Randy Wright <rwright@....com>,
        Jerry Hoemann <jerry.hoemann@....com>,
        Dave Young <dyoung@...hat.com>,
        Khalid Aziz <khalid@...ehiking.org>
Subject: Re: [RFC PATCH] PCI, kdump: Clear bus master bit upon shutdown in
 kdump kernel

On Wed, Jul 22, 2020 at 10:52:26PM +0800, Kairui Song wrote:
> On Fri, Mar 6, 2020 at 5:38 PM Baoquan He <bhe@...hat.com> wrote:
> > On 03/04/20 at 08:53pm, Deepa Dinamani wrote:
> > > On Wed, Mar 4, 2020 at 7:53 PM Baoquan He <bhe@...hat.com> wrote:
> > > > On 03/03/20 at 01:01pm, Deepa Dinamani wrote:
> > > > > I looked at this some more. Looks like we do not clear irqs
> > > > > when we do a kexec reboot. And, the bootup code maintains
> > > > > the same table for the kexec-ed kernel. I'm looking at the
> > > > > following code in
> > > >
> > > > I guess you are talking about kdump reboot here, right? Kexec
> > > > and kdump boot take the similar mechanism, but differ a
> > > > little.
> > >
> > > Right I meant kdump kernel here. And, clearly the
> > > is_kdump_kernel() case below.
> > >
> > > > > intel_irq_remapping.c:
> > > > >
> > > > >         if (ir_pre_enabled(iommu)) {
> > > > >                 if (!is_kdump_kernel()) {
> > > > >                         pr_warn("IRQ remapping was enabled on %s but
> > > > > we are not in kdump mode\n",
> > > > >                                 iommu->name);
> > > > >                         clear_ir_pre_enabled(iommu);
> > > > >                         iommu_disable_irq_remapping(iommu);
> > > > >                 } else if (iommu_load_old_irte(iommu))
> > > >
> > > > Here, it's for kdump kernel to copy old ir table from 1st kernel.
> > >
> > > Correct.
> > >
> > > > >                         pr_err("Failed to copy IR table for %s from
> > > > > previous kernel\n",
> > > > >                                iommu->name);
> > > > >                 else
> > > > >                         pr_info("Copied IR table for %s from previous kernel\n",
> > > > >                                 iommu->name);
> > > > >         }
> > > > >
> > > > > Would cleaning the interrupts(like in the non kdump path
> > > > > above) just before shutdown help here? This should clear the
> > > > > interrupts enabled for all the devices in the current
> > > > > kernel. So when kdump kernel starts, it starts clean. This
> > > > > should probably help block out the interrupts from a device
> > > > > that does not have a driver.
> > > >
> > > > I think stopping those devices out of control from continue
> > > > sending interrupts is a good idea. While not sure if only
> > > > clearing the interrupt will be enough. Those devices which
> > > > will be initialized by their driver will brake, but devices
> > > > which drivers are not loaded into kdump kernel may continue
> > > > acting. Even though interrupts are cleaning at this time, the
> > > > on-flight DMA could continue triggerring interrupt since the
> > > > ir table and iopage table are rebuilt.
> > >
> > > This should be handled by the IOMMU, right? And, hence you are
> > > getting UR. This seems like the correct execution flow to me.
> >
> > Sorry for late reply.
> > Yes, this is initializing IOMMU device.
> >
> > > Anyway, you could just test this theory by removing the
> > > is_kdump_kernel() check above and see if it solves your problem.
> > > Obviously, check the VT-d spec to figure out the exact sequence to
> > > turn off the IR.
> >
> > OK, I will talk to Kairui and get a machine to test it. Thanks for your
> > nice idea, if you have a draft patch, we are happy to test it.
> >
> > > Note that the device that is causing the problem here is a legit
> > > device. We want to have interrupts from devices we don't know about
> > > blocked anyway because we can have compromised firmware/ devices that
> > > could cause a DoS attack. So blocking the unwanted interrupts seems
> > > like the right thing to do here.
> >
> > Kairui said it's a device which driver is not loaded in kdump kernel
> > because it's not needed by kdump. We try to only load kernel modules
> > which are needed, e.g one device is the dump target, its driver has to
> > be loaded in. In this case, the device is more like a out of control
> > device to kdump kernel.
> 
> Hi Bao, Deepa, sorry for this very late response. The test machine was
> not available for sometime, and I restarted to work on this problem.
> 
> For the workaround mention by Deepa (by remote the is_kdump_kernel()
> check), it didn't work, the machine still hangs upon shutdown.
> The devices that were left in an unknown state and sending interrupt
> could be a problem, but it's irrelevant to this hanging problem.
> 
> I think I didn't make one thing clear, The PCI UR error never arrives
> in kernel, it's the iLo BMC on that HPE machine caught the error, and
> send kernel an NMI. kernel is panicked by NMI, I'm still trying to
> figure out why the NMI hanged kernel, even with panic=-1,
> panic_on_io_nmi, panic_on_unknown_nmi all set. But if we can avoid the
> NMI by shutdown the devices in right order, that's also a solution.

I'm not sure how much sympathy to have for this situation.  A PCIe UR
is fatal for the transaction and maybe even the device, but from the
overall system point of view, it *should* be a recoverable error and
we shouldn't panic.

Errors like that should be reported via the normal AER or ACPI/APEI
mechanisms.  It sounds like in this case, the platform has decided
these aren't enough and it is trying to force a reboot?  If this is
"special" platform behavior, I'm not sure how much we need to cater
for it.

Bjorn

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