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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Mon, 7 Nov 2022 08:54:54 +0100
Message-ID: <CAMj1kXEyR=fwYO2z3w8C+otpxB33owv6fEBTXs_-++Ln=P4g8w@mail.gmail.com>
Subject: Re: Bug: kexec on Lenovo ThinkPad T480 disables EFI mode
To:     Dave Young <dyoung@redhat.com>
Cc:     ns@tfwno.gf, Baoquan He <bhe@redhat.com>,
        Eric Biederman <ebiederm@xmission.com>,
        kexec@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-efi@vger.kernel.org
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On Mon, 7 Nov 2022 at 08:40, Dave Young <dyoung@redhat.com> wrote:
>
> On Mon, 7 Nov 2022 at 15:36, Dave Young <dyoung@redhat.com> wrote:
> >
> > Hi Ard,
> >
> > On Mon, 7 Nov 2022 at 15:30, Ard Biesheuvel <ardb@kernel.org> wrote:
> > >
> > > On Mon, 7 Nov 2022 at 07:55, Dave Young <dyoung@redhat.com> wrote:
> > > >
> > > > Hi,
> > > >
> > > > On Sat, 5 Nov 2022 at 22:16, <ns@tfwno.gf> wrote:
> > > > >
> > > > > On 2022-11-05 05:49, Dave Young wrote:
> > > > > > Baoquan, thanks for cc me.
> > > > > >
> > > > > > On Sat, 5 Nov 2022 at 11:10, Baoquan He <bhe@redhat.com> wrote:
> > > > > >>
> > > > > >> Add Dave to CC
> > > > > >>
> > > > > >> On 10/28/22 at 01:02pm, ns@tfwno.gf wrote:
> > > > > >> > Greetings,
> > > > > >> >
> > > > > >> > I've been hitting a bug on my Lenovo ThinkPad T480 where kexecing will
> > > > > >> > cause EFI mode (if that's the right term for it) to be unconditionally
> > > > > >> > disabled, even when not using the --noefi option to kexec.
> > > > > >> >
> > > > > >> > What I mean by "EFI mode" being disabled, more than just EFI runtime
> > > > > >> > services, is that basically nothing about the system's EFI is visible
> > > > > >> > post-kexec. Normally you have a message like this in dmesg when the
> > > > > >> > system is booted in EFI mode:
> > > > > >> >
> > > > > >> > [    0.000000] efi: EFI v2.70 by EDK II
> > > > > >> > [    0.000000] efi: SMBIOS=0x7f98a000 ACPI=0x7fb7e000 ACPI 2.0=0x7fb7e014
> > > > > >> > MEMATTR=0x7ec63018
> > > > > >> > (obviously not the real firmware of the machine I'm talking about, but I
> > > > > >> > can also send that if it would be of any help)
> > > > > >> >
> > > > > >> > No such message pops up in my dmesg as a result of this bug, & this
> > > > > >> > causes some fallout like being unable to find the system's DMI
> > > > > >> > information:
> > > > > >> >
> > > > > >> > <6>[    0.000000] DMI not present or invalid.
> > > > > >> >
> > > > > >> > The efivarfs module also fails to load with -ENODEV.
> > > > > >> >
> > > > > >> > I've tried also booting with efi=runtime explicitly but it doesn't
> > > > > >> > change anything. The kernel still does not print the name of the EFI
> > > > > >> > firmware, DMI is still missing, & efivarfs still fails to load.
> > > > > >> >
> > > > > >> > I've been using the kexec_load syscall for all these tests, if it's
> > > > > >> > important.
> > > > > >> >
> > > > > >> > Also, to make it very clear, all this only ever happens post-kexec. When
> > > > > >> > booting straight from UEFI (with the EFI stub), all the aforementioned
> > > > > >> > stuff that fails works perfectly fine (i.e. name of firmware is printed,
> > > > > >> > DMI is properly found, & efivarfs loads & mounts just fine).
> > > > > >> >
> > > > > >> > This is reproducible with a vanilla 6.1-rc2 kernel. I've been trying to
> > > > > >> > bisect it, but it seems like it goes pretty far back. I've got vanilla
> > > > > >> > mainline kernel builds dating back to 5.17 that have the exact same
> > > > > >> > issue. It might be worth noting that during this testing, I made sure
> > > > > >> > the version of the kernel being kexeced & the kernel kexecing were the
> > > > > >> > same version. It may not have been a problem in older kernels, but that
> > > > > >> > would be difficult to test for me (a pretty important driver for this
> > > > > >> > machine was only merged during v5.17-rc4). So it may not have been a
> > > > > >> > regression & just a hidden problem since time immemorial.
> > > > > >> >
> > > > > >> > I am willing to test any patches I may get to further debug or fix
> > > > > >> > this issue, preferably based on the current state of torvalds/linux.git.
> > > > > >> > I can build & test kernels quite a few times per day.
> > > > > >> >
> > > > > >> > I can also send any important materials (kernel config, dmesg, firmware
> > > > > >> > information, so on & so forth) on request. I'll also just mention I'm
> > > > > >> > using kexec-tools 2.0.24 upfront, if it matters.
> > > > > >
> > > > > > Can you check the efi runtime in sysfs:
> > > > > > ls /sys/firmware/efi/runtime-map/
> > > > > >
> > > > > > If nothing then maybe you did not enable CONFIG_EFI_RUNTIME_MAP=y, it
> > > > > > is needed for kexec UEFI boot on x86_64.
> > > > >
> > > > > Oh my, it really is that simple.
> > > > >
> > > > > Indeed, enabling this in the pre-kexec kernel fixes it all up. I had
> > > > > blindly disabled it in my quest to downsize the pre-kexec kernel to
> > > > > reduce boot time (it only runs a bootloader). In hindsight, the firmware
> > > > > drivers section is not really a good section to tweak on a whim.
> > > > >
> > > > > I'm terribly sorry to have taken your time to "fix" this "bug". But I
> > > > > must ask, is there any reason why this is a visible config option, or at
> > > > > least not gated behind CONFIG_EXPERT? drivers/firmware/efi/runtime-map.c
> > > > > is pretty tiny, & considering it depends on CONFIG_KEXEC_CORE, one
> > > > > probably wants to have kexec work properly if they can even enable it.
> > > >
> > > > Glad to know it works with the .config tweaking. I can not recall any
> > > > reason for that though.
> > > >
> > > > Since it sits in the efi code path, let's see how Ard thinks about
> > > > your proposal.
> > > >
> > >
> > > I don't understand why EFI_RUNTIME_MAP should depend on KEXEC_CORE at
> > > all: it is documented as a feature that can be enabled for debugging
> > > as well, and kexec does not work as expected without it.
> >
> > Probably debugging only mentioned in text, but not been considered in
> > the kconfig logic :(
> >
> > >
> > > Should we just change it like this perhaps?
> > >
> > > --- a/drivers/firmware/efi/Kconfig
> > > +++ b/drivers/firmware/efi/Kconfig
> > > @@ -28,8 +28,8 @@ config EFI_VARS_PSTORE_DEFAULT_DISABLE
> > >
> > >  config EFI_RUNTIME_MAP
> > >         bool "Export efi runtime maps to sysfs"
> > > -       depends on X86 && EFI && KEXEC_CORE
> > > -       default y
> > > +       depends on X86 && EFI
> > > +       default KEXEC_CORE
> > >         help
> > >
> > > and maybe add an 'if EXPERT' so that the option is only visible to
> > > modify when CONFIG_EXPERT=y ?
> >
> > Above changes look good to me.
> >
> > >
> > > In any case, I intend to move this code into arch/x86 as well, so I'll
> > > have a couple of patches out shortly.
> >
> > That would be better since it is X86 only.  Thanks, Ard.
>
> Hmm,  before doing that, do you think it is useful for debugging
> purposes? That could be a reason to sit in efi code instead of x86 ..
>

This code was only ever enabled on x86, and on ARM/arm64, we can
capture the memory map via efi=debug on any kernel build, and capture
the virtual mappings using PTDUMP (which also gives us the exact
attributes for each mapped region)

So I don't think it has that much value on non-x86 tbh.