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

[Fenghua Yu]

Hi, 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.

Please advice.

Thanks.

-Fenghua