Re: [Regression] kdump fails to get DHCP address unless booting with pci=nomsi or without nr_cpus=1

[Marc Zyngier <maz@kernel.org>]

On Tue, 12 Aug 2025 11:09:12 +0100,
Coiby Xu <coxu@redhat.com> wrote:
> 
> On Mon, Aug 11, 2025 at 03:52:04PM +0100, Marc Zyngier wrote:
> > On Mon, 11 Aug 2025 14:03:21 +0100,
> > Thomas Gleixner <tglx@linutronix.de> wrote:
> >> 
> >> On Mon, Aug 11 2025 at 15:02, Thomas Gleixner wrote:
> >> 
> >> CC+ Marc
> >> 
> >> > On Mon, Aug 11 2025 at 11:23, Coiby Xu wrote:
> >> >> Recently I met an issue that on certain virtual machines, the kdump
> >> >> kernel fails to get DHCP IP address most of times starting from
> >> >> 6.11-rc2. git bisection shows commit b5712bf89b4b ("irqchip/gic-v3-its:
> >> >> Provide MSI parent for PCI/MSI[-X]") is the 1st bad commit,
> >> >>
> >> >>      # good: [7d189c77106ed6df09829f7a419e35ada67b2bd0] PCI/MSI: Provide
> >> >>      # MSI_FLAG_PCI_MSI_MASK_PARENT
> >> >>      git bisect good 7d189c77106ed6df09829f7a419e35ada67b2bd0
> >> >>      # good: [48f71d56e2b87839052d2a2ec32fc97a79c3e264] irqchip/gic-v3-its:
> >> >>      # Provide MSI parent infrastructure
> >> >>      git bisect good 48f71d56e2b87839052d2a2ec32fc97a79c3e264
> >> >>      # good: [8c41ccec839c622b2d1be769a95405e4e9a4cb20] irqchip/irq-msi-lib:
> >> >>      # Prepare for PCI MSI/MSIX
> >> >>      git bisect good 8c41ccec839c622b2d1be769a95405e4e9a4cb20
> >> >>      # first bad commit: [b5712bf89b4bbc5bcc9ebde8753ad222f1f68296]
> >> >>      # irqchip/gic-v3-its: Provide MSI parent for PCI/MSI[-X]
> >> >
> >> > There were follow up fixes on this, so isolating this one is not really
> >> > conclusive.
> >> >
> >> > Is the problem still there on v6.16 and v6.17-rc1?
> > 
> > Yeah, there are way too many things that have been addressed since.
> > kdump is also a particularly nasty case, as it tends to rely on the
> > redistributor tables programmed by the previous kernel.
> 
> Thanks for providing a clue. This may also explain explain why I fails
> to reproduce this issue against 1st kernel even with the same cmdline of
> the kdump kernel.

I'm not sure that's a clue. It's only an indication that things are
not necessarily easy to spot.

Has it ever been reproduced on bare metal? Have you tried v6.16 as
instructed?

> 
> > 
> > Also, this says "virtual machines". What's the hypervisor? 
> 
> I'll contact the lab administrator. What kinds of info I should collect
> to help you narrow down the issue?

Surely you know what hypervisor you're running on, right?

> 
> > How hard is it to reproduce?
> 
> It can be reproduced reliably on certain machines. But as of writing I
> haven't reproduced it on other KVM virtual machines on three different
> host machines.

Which machines? I'm sorry, but if you want help on this, you'll have
to provide actual information.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.