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

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Date:   Wed, 4 Mar 2020 22:06:58 -0800
From:   Deepa Dinamani <deepa.kernel@...il.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     Kairui Song <kasong@...hat.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Khalid Aziz <khalid@...ehiking.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pci@...r.kernel.org, kexec@...ts.infradead.org,
        Jerry Hoemann <jerry.hoemann@....com>,
        Randy Wright <rwright@....com>, Dave Young <dyoung@...hat.com>,
        Myron Stowe <myron.stowe@...hat.com>, jroedel@...e.de
Subject: Re: [RFC PATCH] PCI, kdump: Clear bus master bit upon shutdown in
 kdump kernel

On Wed, Mar 4, 2020 at 8:53 PM Deepa Dinamani <deepa.kernel@...il.com> wrote:
>
> On Wed, Mar 4, 2020 at 7:53 PM Baoquan He <bhe@...hat.com> wrote:
> >
> > +Joerg to CC.
> >
> > 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.

One small correction, I meant the IOMMU and BME here.