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Re: [PATCH] x86/cpu: Drop _init from *_cpu_cap functions
- To: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Thu, 11 Aug 2022 10:34:55 +0000
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- Cc: Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
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- Thread-topic: [PATCH] x86/cpu: Drop _init from *_cpu_cap functions
On 11/08/2022 11:30, Ross Lagerwall wrote:
>> From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
>> Sent: Thursday, August 11, 2022 11:21 AM
>> To: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>;
>> xen-devel@xxxxxxxxxxxxxxxxxxxx <xen-devel@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Jan Beulich <jbeulich@xxxxxxxx>; Roger Pau Monne <roger.pau@xxxxxxxxxx>;
>> Wei Liu <wl@xxxxxxx>
>> Subject: Re: [PATCH] x86/cpu: Drop _init from *_cpu_cap functions
>>
>> On 11/08/2022 11:17, Ross Lagerwall wrote:
>>> These functions may be called by init_amd() after the _init functions
>>> have been purged during CPU hotplug or PV shim boot so drop the _init.
>>>
>>> Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
>> Hmm. That's a bug in init_amd() I'd say. These really shouldn't be
>> used after __init.
>>
>> Which path exploded specifically?
> The stack trace was:
>
> setup_force_cpu_cap
> init_amd
> identify_cpu
> start_secondary
>
> In setup_force_cpu_cap() here:
>
> /*
> * On pre-CLFLUSHOPT AMD CPUs, CLFLUSH is weakly ordered with
> * everything, including reads and writes to address, and
> * LFENCE/SFENCE instructions.
> */
> if (!cpu_has_clflushopt)
> setup_force_cpu_cap(X86_BUG_CLFLUSH_MFENCE);
>
> which was recently introduced by:
>
> commit 062868a5a8b428b85db589fa9a6d6e43969ffeb9
> Author: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Date: Thu Jun 9 14:23:07 2022 +0200
>
> x86/amd: Work around CLFLUSH ordering on older parts
Bah, and that was also backported in a security fix, to everything back
to 4.12 is broken.
> Should the fix rather be to guard that call with "if (c == &boot_cpu_data
> ..." ?
Yes please.
Sorry.
~Andrew
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