Re: [PATCH v2 12/12] x86/traps: Fix up invalid PASID

[Andy Lutomirski]

> On Jun 15, 2020, at 1:17 PM, Fenghua Yu <fenghua.yu@xxxxxxxxx> wrote:
> 
> ïHi, Peter,
> 
>> On Mon, Jun 15, 2020 at 09:09:28PM +0200, Peter Zijlstra wrote:
>>> On Mon, Jun 15, 2020 at 11:55:29AM -0700, Fenghua Yu wrote:
>>> 
>>> Or do you suggest to add a random new flag in struct thread_info instead
>>> of a TIF flag?
>> 
>> Why thread_info? What's wrong with something simple like the below. It
>> takes a bit from the 'strictly current' flags word.
>> 
>> 
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index b62e6aaf28f0..fca830b97055 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -801,6 +801,9 @@ struct task_struct {
>>    /* Stalled due to lack of memory */
>>    unsigned            in_memstall:1;
>> #endif
>> +#ifdef CONFIG_PCI_PASID
>> +    unsigned            has_valid_pasid:1;
>> +#endif
>> 
>>    unsigned long            atomic_flags; /* Flags requiring atomic access. */
>> 
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 142b23645d82..10b3891be99e 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -955,6 +955,10 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
>>    tsk->use_memdelay = 0;
>> #endif
>> 
>> +#ifdef CONFIG_PCI_PASID
>> +    tsk->has_valid_pasid = 0;
>> +#endif
>> +
>> #ifdef CONFIG_MEMCG
>>    tsk->active_memcg = NULL;
>> #endif
> 
> The PASID MSR is x86 specific although PASID is PCIe concept and per-mm.
> Checking if the MSR has valid PASID (bit31=1) is an x86 specifc work.
> The flag should be cleared in cloned()/forked() and is only set and
> read in fixup() in x86 #GP for heuristic. It's not used anywhere outside
> of x86.
> 
> That's why we think the flag should be in x86 struct thread_info instead
> of in generice struct task_struct.
> 

Are we planning to keep PASID live once a task has used it once or are we going to swap it lazily?  If the latter, a percpu variable might be better.

> Please advice.
> 
> Thanks.
> 
> -Fenghua