Re: [PATCH 15/24] x86/mm: Allow flushing for future ASID switches

Peter Zijlstra
2017-11-28T19:05:17Z

On Tue, Nov 28, 2017 at 10:13:30AM -0800, Dave Hansen wrote:
> Thanks for looking at this, Peter. I've been resisting doing this for a
> bit and it's an embarrassingly small amount of code.

Right, well, its not complete yet, and it might be complete crap :-)

> On 11/28/2017 08:39 AM, Peter Zijlstra wrote:
> > @@ -220,7 +221,21 @@ For 32-bit we have the following conventions - kernel is built with
> > .macro SWITCH_TO_USER_CR3 scratch_reg:req
> > STATIC_JUMP_IF_FALSE .Lend_\@, kaiser_enabled_key, def=1
> > mov %cr3, \scratch_reg
> > - ADJUST_USER_CR3 \scratch_reg
> > + push \scratch_reg
>
> Do we have a good stack in all the spots that we need to do this? It
> may have changed with the trampoline stack, but I'm 100% sure that it
> wasn't so in the recent past.

Dunno really. I figured I'd give it a go and see what happens. So far
the machine still works. But I was hoping Andy would have an opinion on
this.

> Let me see if I'm reading the assembly right.

Yep, seems you can read asm :-)

> > +DECLARE_PER_CPU(unsigned long, __asid_flush);
>
> Could we spare enough space to make this something like
> user_asid_flush_pending_mask?

Yeah, if I can get it all working we'll bikeshed on a name ;-)

> It took me a minute to realize that it was a mask. Also, since we only
> have 6 asids, should we bit a bit more stingy with the type?

I picked unsigned long because our bitops (__set_bit in this case, use
it), and I know we're LE and could simply use a shorter type, but meh.

> It took me a minute to realize that mixing these is still OK, even if
> the mm associated with the ASID changes. It's because once the ASID is
> stale, it doesn't matter *why* it is stale. Just that the next guy who
> *uses* it needs to do the flush. You can do 1,000 tlb flushes, a
> context switch, a tlb flush and another context switch, but if you only
> go out to userspace once, you only need 1 ASID flush. That fits
> perfectly with this bit that gets set a bunch of times and only cleared
> once at exit to userspace.

Just so.

I'm now staring at the RESTORE_CR3 stuff, and that appears to be called
in the NMI handling where the stack is not to be used (if I read it
right), so that's going to be a little more tricky.

Let me prod at that..