Message-ID:
 <SN6PR02MB41574DC1576DC20772D9CB05D4F22@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Mon, 10 Feb 2025 21:35:40 +0000
From: Michael Kelley <mhklinux@...look.com>
To: "thomas.tai@...cle.com" <thomas.tai@...cle.com>, "mhkelley58@...il.com"
	<mhkelley58@...il.com>, "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
	"wei.liu@...nel.org" <wei.liu@...nel.org>, "decui@...rosoft.com"
	<decui@...rosoft.com>, "drawat.floss@...il.com" <drawat.floss@...il.com>,
	"javierm@...hat.com" <javierm@...hat.com>, Helge Deller <deller@....de>,
	"daniel@...ll.ch" <daniel@...ll.ch>, "airlied@...il.com" <airlied@...il.com>,
	"tzimmermann@...e.de" <tzimmermann@...e.de>
CC: "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Subject: RE: hyper_bf soft lockup on Azure Gen2 VM when taking kdump or
 executing kexec

From: thomas.tai@...cle.com <thomas.tai@...cle.com> Sent: Monday, February 10, 2025 7:08 AM
> 
> <snip>
> 
> >> Then the question is why the efifb driver doesn't work in the kdump
> >> kernel. Actually, it *does* work in many cases. I built the 6.13.0 kernel
> >> on the Oracle Linux 9.4 system, and transferred the kernel image binary
> >> and module binaries to an Ubuntu 20.04 VM in Azure. In that VM, the
> >> efifb driver is loaded as part of the kdump kernel, and it doesn't cause
> >> any problems. But there's an interesting difference. In the Oracle Linux
> >> 9.4 VM, the efifb driver finds the framebuffer at 0x40000000, while on
> >> the Ubuntu 20.04 VM, it finds the framebuffer at 0x40900000. This
> >> difference is due to differences in how the screen_info variable gets
> >> setup in the two VMs.
> >>
> >> When the normal kernel starts in a freshly booted VM, Hyper-V provides
> >> the EFI framebuffer at 0x40000000, and it works. But after the Hyper-V
> >> FB driver or Hyper-V DRM driver has initialized, Linux has picked a
> >> different MMIO address range and told Hyper-V to use the new
> >> address range (which often starts at 0x40900000). A kexec does *not*
> >> reset Hyper-V's transition to the new range, so when the efifb driver
> >> tries to use the framebuffer at 0x40000000, the accesses trap to
> >> Hyper-V and probably fail or timeout (I'm not sure of the details). After
> >> the guest does some number of these bad references, Hyper-V considers
> >> itself to be under attack from an ill-behaving guest, and throttles the
> >> guest so that it doesn't run for a few seconds. The throttling repeats,
> >> and results in extremely slow running in the kdump kernel.
> >>
> >> Somehow in the Ubuntu 20.04 VM, the location of the frame buffer
> >> as stored in screen_info.lfb_base gets updated to be 0x40900000. I
> >> haven't fully debugged how that happens. But with that update, the
> >> efifb driver is using the updated framebuffer address and it works. On
> >> the Oracle Linux 9.4 system, that update doesn't appear to happen,
> >> and the problem occurs.
> >>
> >> This in an interim update on the problem. I'm still investigating how
> >> screen_info.lfb_base is set in the kdump kernel, and why it is different
> >> in the Ubuntu 20.04 VM vs. in the Oracle Linux 9.4 VM. Once that is
> >> well understood, we can contemplate how to fix the problem. Undoing
> >> the revert that is commit 2bebc3cd4870 doesn't seem like the solution
> >> since the original code there was reported to cause many other issues.
> >> The solution focus will likely be on how to ensure the kdump kernel gets
> >> the correct framebuffer address so the efifb driver works, since the
> >> framebuffer address changing is a quirk of Hyper-V behavior.
> >>
> >> If anyone else has insight into what's going on here, please chime in.
> >> What I've learned so far is still somewhat tentative.
> >>
> > Here's what is happening. On Ubuntu 20.04, the kdump image is
> > loaded into crash memory using the kexec command. Ubuntu 20.04
> > has kexec from the kexec-tools package version 2.0.18-1ubuntu1.1,
> > and per the kexec man page, it defaults to using the older kexec_load()
> > system call. When using kexec_load(), the contents to be loaded into
> > crash memory is constructed in user space by the kexec command.
> > The kexec command gets the "screen_info" settings, including the
> > physical address of the frame buffer, via the FBIOGET_FSCREENINFO
> > ioctl against /dev/fb0. The Hyper-V FB or DRM driver registers itself
> > with the fbdev subsystem so that it is /dev/fb0, and the ioctl returns
> > the updated framebuffer address. So the efifb driver loads and runs
> > correctly.
> >
> > On Oracle Linux 9.4, the kdump image is also loaded with the
> > kexec command, but from kexec-tools package version
> > kexec-tools-2.0.28-1.0.10.el9_5.x86_64, which is slightly later than
> > the version on Ubuntu 20.04. This newer kexec defaults to using the
> > newer kexec_file_load() system call. This system call gets the
> > framebuffer address from the screen_info variable in the kernel, which
> > has not been updated to reflect the new framebuffer address. Hence
> > in the kdump kernel, the efifb driver uses the old framebuffer address,
> > and hence the problem.
> >
> > To further complicate matters, the kexec on Oracle Linux 9.4 seems to
> > have a bug when the -c option forces the use of kexec_load() instead
> > of kexec_file_load(). As an experiment, I modified the kdumpctl shell
> > script to add the "-c" option to kexec, but in that case the value "0x0"
> > is passed as the framebuffer address, which is wrong. Furthermore,
> > the " screen_info.orig_video_isVGA" value (which I mentioned earlier
> > in connection with commit 2bebc3cd4870) is also set to 0, so the
> > kdump kernel no longer thinks it has an EFI framebuffer. Hence the
> > efifb driver isn't loaded, and the kdump works, though for the wrong
> > reasons. If kexec 2.0.18 from Ubuntu is copied onto the Oracle Linux 9.4
> > VM, then kdump works as expected, with the efifb driver being loaded
> > and using the correct framebuffer address. So something is going wrong
> > with kexec 2.0.28 in how it sets up the screen_info when the -c option
> > is used. I'll leave the debugging of the kexec bug to someone else.
>
> Hi Michael,
> 
> Do you think we need to handle Azure Gen2 VM differently in the kexec?
> 
> Or should we change the kexec_file_load() system call to retrieve the correct
> framebuffer address?

I'm thinking there may be a fix in the Hyper-V FB and Hyper-V DRM drivers.
Commit c25a19afb81c may also be a cause of the problem -- see precursor
commit 3cb73bc3fa2a, which describes exactly the problem. I still need to
do some testing, but without that commit, kdump won't detect that it has
an EFI framebuffer, won't load the efifb driver, and so won't encounter the
problem. But we probably need to get Thomas Zimmerman to weigh in on
the implications of reverting c25a19afb81c.

There's one additional variation of the problem. Assume the Hyper-V FB
driver is loaded (for example) during boot and moves the framebuffer. Then
system runs kexec as part of arming kdump during the boot sequence.
The most recent location of the framebuffer (and whether it is an EFI framebuffer)
gets picked at the time kexec runs, and is stored in the crash kernel memory area.
But what if the framebuffer later moves, perhaps because the Hyper-V FB driver
is unbound? The crash kernel memory area doesn’t get updated and kdump
could still have the wrong framebuffer address. This anomaly argues for the
commit 3cb73bc3fa2a approach of just ensuring that the efifb driver doesn't
load. Of course that approach means that the kdump kernel *must* contain
either the Hyper-V FB or Hyper-V DRM driver in order to work on a system
with only a framebuffer for text output. The efifb driver won't work. But
perhaps that's OK.

Changing kexec (or the invoking script) to special case Hyper-V Gen 2 VMs and
always use kexec_load() instead of kexec_file_load() sounds like a big hack
to me.  And with that approach, you give up the ability to enforce loading only
properly signed kdump images. This is something kexec_file_load() provides
that kexec_load() doesn't, and is one of the main reasons that kexec_file_load()
was added.

Whether the kexec_file_load() system call could be enhanced to get the
frame buffer information from the /dev/fb0 device, I'm not sure. That might
be a reasonable approach, though it still has the problem that the framebuffer
address could change *after* kexec_file_load() runs.

Anyway, that's a dump of my current thoughts. I haven't reached a final
conclusion or recommendation yet. Comments from others on the
thread are welcome.

Michael

> 
> Thank you,
> Thomas
>
> > I'm still thinking about alternatives to fix this mess. Please chime
> > in if you have suggestions.
> >
> > Michael

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