Re: [RFC PATCH v2 16/16] [DO NOT MERGE] x86/kexec: enable DEBUG

[David Woodhouse <dwmw2@infradead.org>]

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On Mon, 2024-11-25 at 21:34 +0100, Ingo Molnar wrote:
>  
> > The realisation that we never even explicitly mapped the control code 
> > page and always just got lucky because it happened to be in the same 
> > 2MiB or 1GiB superpage as something else that we did map... was just 
> > a bonus :)
> 
> I'm amazed and horrified in equal measure ;-)

:)

The rest of today was dedicated to finding out that that isn't entirely
true. Mapping the control page explicitly was only helping because it
forced 2MiB mappings instead of a 1GiB mapping, and masked the fact
that PTI was causing the identmap code to scribble off the end of the
root PGD page...

It all just worked by pure fluke on x86_64 before, because x86_64 would
allocate a 8KiB control region and use the first half of it for the
PGD, and *then* copy the trampoline code into the second half, after
the identmap code had finished scribbling on it. So when I cleaned that
up to allocate the PGD separately and explicitly like i386 does, that's
why it exploded; not just due to allocation patterns.

Still, I think I have a handle on fairly much everything that's broken,
except the occasional warning on the way back from
KEXEC_PRESERVE_CONTEXT thus:

[    1.423464] ------------[ cut here ]------------
[    1.423950] Interrupts enabled after irqrouter_resume+0x0/0x50
[    1.424605] WARNING: CPU: 0 PID: 215 at drivers/base/syscore.c:103 syscore_resume+0x152/0x180
[    1.425467] Modules linked in:
[    1.425791] CPU: 0 UID: 0 PID: 215 Comm: kexec Not tainted 6.12.0-rc5+ #2015
[    1.426498] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[    1.427628] RIP: 0010:syscore_resume+0x152/0x180
[    1.428101] Code: 00 e9 e1 fe ff ff 80 3d b1 b8 c4 01 00 0f 85 21 ff ff ff 48 8b 73 18 48 c7 c7 32 b8 b6 ac c6 05 99 b8 c4 01 01 e8 9e 3f 55 ff <0f> 0b e9 03 ff ff ff 80 3d 87 b8 c4 01 00 0f 85 b8 fe ff ff 48 c7
[    1.429913] RSP: 0018:ffffae9bc03bfd00 EFLAGS: 00010282
[    1.430445] RAX: 0000000000000000 RBX: ffffffffad6fbb20 RCX: ffffffffad5636a8
[    1.431153] RDX: 0000000000000000 RSI: 0000000000000003 RDI: 0000000000000001
[    1.431869] RBP: 0000000028121969 R08: 0000000000000000 R09: 0000000000000000
[    1.432594] R10: ffffae9bc03bfaa8 R11: 7075727265746e49 R12: ffffae9bc03bfd28
[    1.433313] R13: ffffffffad471f60 R14: 00000000fee1dead R15: 0000000000000000
[    1.434021] FS:  00007f77d4a45740(0000) GS:ffff91d0fd600000(0000) knlGS:0000000000000000
[    1.434815] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    1.435385] CR2: 00007f7e011e7070 CR3: 00000000012fe001 CR4: 0000000000170ef0
[    1.436073] Call Trace:
[    1.436334]  <TASK>
[    1.436558]  ? syscore_resume+0x152/0x180
[    1.436956]  ? __warn.cold+0x93/0xfa
[    1.437319]  ? syscore_resume+0x152/0x180
[    1.437717]  ? report_bug+0xff/0x140
[    1.438075]  ? handle_bug+0x58/0x90
[    1.438438]  ? exc_invalid_op+0x17/0x70
[    1.438826]  ? asm_exc_invalid_op+0x1a/0x20
[    1.439246]  ? syscore_resume+0x152/0x180
[    1.439644]  kernel_kexec+0x10a/0x160
[    1.440010]  __do_sys_reboot+0x1fd/0x240
[    1.440485]  do_syscall_64+0x82/0x160
[    1.440863]  ? syscall_exit_to_user_mode+0x10/0x210
[    1.441351]  ? do_syscall_64+0x8e/0x160
[    1.441735]  ? exc_page_fault+0x7e/0x180
[    1.442123]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[    1.442623] RIP: 0033:0x7f77d4b5adb7
[    1.442992] Code: c7 c0 ff ff ff ff eb be 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 89 fa be 69 19 12 28 bf ad de e1 fe b8 a9 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 01 c3 48 8b 15 49 50 0c 00 f7 d8 64 89 02 b8
[    1.444757] RSP: 002b:00007ffc56bc30f8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a9
[    1.445493] RAX: ffffffffffffffda RBX: 00007ffc56bc3260 RCX: 00007f77d4b5adb7
[    1.446173] RDX: 0000000045584543 RSI: 0000000028121969 RDI: 00000000fee1dead
[    1.446848] RBP: 00007ffc56bc32c0 R08: 000055e1cef3e010 R09: 0000000000000007
[    1.447527] R10: 000055e1cef41020 R11: 0000000000000246 R12: 0000000000000001
[    1.448219] R13: 000055e19046b896 R14: 000055e1cef3e4a0 R15: 0000000000000000
[    1.448893]  </TASK>
[    1.449119] ---[ end trace 0000000000000000 ]---
[    1.452539] Enabling non-boot CPUs ...
[    1.452935] crash hp: kexec_trylock() failed, kdump image may be inaccurate
[    1.453678] smpboot: Booting Node 0 Processor 1 APIC 0x1
[    1.455531] CPU1 is up
[    1.460031] virtio_blk virtio1: 2/0/0 default/read/poll queues
[    1.465246] OOM killer enabled.
[    1.465580] Restarting tasks ... done.



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