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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jun 15, 2020 at 01:17:35PM -0700, Fenghua Yu wrote:
> 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.

I don't think anybody really cares, it's just one bit, we have plenty
left.

On x86_64 there's a u32 sized alignment hole in thread_info, also we
don't use the upper 32bit of thread_info::flags, however using those
would still mean you have to use atomic ops, which you really don't
need.