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Re: [PATCH] x86/cpu: revert opt_allow_unsafe from __ro_after_init to __read_mostly
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 2 Sep 2024 17:16:05 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 02 Sep 2024 15:16:16 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 02.09.2024 17:00, Roger Pau Monne wrote:
> Making opt_allow_unsafe read only after init requires changes to the logic in
> init_amd(), otherwise the following #PF happens on CPU hotplug:
>
> ----[ Xen-4.20.0-1-d x86_64 debug=y Tainted: H ]----
> CPU: 1
> RIP: e008:[<ffff82d040291081>] arch/x86/cpu/amd.c#init_amd+0x37f/0x993
> [...]
> Xen call trace:
> [<ffff82d040291081>] R arch/x86/cpu/amd.c#init_amd+0x37f/0x993
> [<ffff82d040291fbe>] F identify_cpu+0x2d4/0x4db
> [<ffff82d04032eeaa>] F start_secondary+0x22e/0x3cf
> [<ffff82d040203327>] F __high_start+0x87/0xa0
>
> Pagetable walk from ffff82d0404011ea:
> L4[0x105] = 000000006fc2e063 ffffffffffffffff
> L3[0x141] = 000000006fc2b063 ffffffffffffffff
> L2[0x002] = 000000807c7ca063 ffffffffffffffff
> L1[0x001] = 800000006f801121 ffffffffffffffff
>
> ****************************************
> Panic on CPU 1:
> FATAL PAGE FAULT
> [error_code=0003]
> Faulting linear address: ffff82d0404011ea
> ****************************************
Hmm, I specifically looked at that code, but I can see how I screwed up.
> For the time being revert opt_allow_unsafe to be __read_mostly.
There's exactly one write that an AP can hit. Is it really worth moving
back, rather than just doing
if (opt_allow_unsafe <= 0 && !cpu_has_amd_erratum(c, AMD_ERRATUM_121))
opt_allow_unsafe = 1;
else if ...
?
Jan
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