Re: [PATCH v2] x86/virt/tdx: Fix lockdep assertion failure in cache flush for kexec

[Nikolay Borisov <nik.borisov@suse.com>]

On 5.03.26 г. 23:35 ч., Huang, Kai wrote:
> 
>>>
>>> The real requirement is tdx_cpu_flush_cache_for_kexec() must be done on
>>> the same CPU.  It's OK that it can be preempted in the middle as long as
>>> it won't be rescheduled to another CPU.
>>
>> TLDR: It wants migration disabled.
> 
> Basically yes.
> 
>>
>>>
>>> Remove the too strong lockdep_assert_preemption_disabled(), and change
>>> this_cpu_{read|write}() to __this_cpu_{read|write}() which provide the more
>>> proper check (when CONFIG_DEBUG_PREEMPT is true), which checks all
>>> conditions that the context cannot be moved to another CPU to run in the
>>> middle.
>>>
>>> Fixes: 61221d07e815 ("KVM/TDX: Explicitly do WBINVD when no more TDX SEAMCALLs")
>>> Cc: stable@vger.kernel.org
>>> Reported-by: Vishal Verma <vishal.l.verma@intel.com>
>>> Signed-off-by: Kai Huang <kai.huang@intel.com>
>>> Tested-by: Vishal Verma <vishal.l.verma@intel.com>
>>
>>
>> So how exactly does this patch prevent the BUG: printk in
>> check_preemption_disabled from triggering, if the lockdep assert was
>> triggering?
> 
> There's no real BUG here.  It's just the
> lockdep_assert_preemption_disabled() is misused.

Essentially in check_preemption_disabled() the check is considered 
passed IF ANY of the preempt disable conditions is met, i.e it's more 
laxed. So yeah, makes sense!

Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>