Re: [PATCH] x86/cpu: revert opt_allow_unsafe from __ro_after_init to __read_mostly

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.09.2024 17:30, Roger Pau Monné wrote:
> On Mon, Sep 02, 2024 at 05:16:05PM +0200, Jan Beulich wrote:
>> 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 ...
>>
>> ?
> 
> I would rather move this logic so it's only the BSP that can set
> opt_allow_unsafe, and the APs check they match the configuration set
> by the BSP.  I think the resulting logic would be cleaner, but I
> didn't want to do such a change as part of this fix.

Well, okay then:
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

And I guess I'll put it in right away.

Jan