Re: [RFC PATCH 2/3] x86/mm: make sure LAM is up-to-date during context switching

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Mar 07, 2024 at 05:34:21PM -0800, Andy Lutomirski wrote:
> Catching up a bit...
> 
> On Thu, Mar 7, 2024, at 5:39 AM, Yosry Ahmed wrote:
> > During context switching, if we are not switching to new mm and no TLB
> > flush is needed, we do not write CR3. However, it is possible that a
> > user thread enables LAM while a kthread is running on a different CPU
> > with the old LAM CR3 mask. If the kthread context switches into any
> > thread of that user process, it may not write CR3 with the new LAM mask,
> > which would cause the user thread to run with a misconfigured CR3 that
> > disables LAM on the CPU.
> 
> So I think (off the top of my head -- haven't thought about it all that hard) that LAM is logically like PCE and LDT: it's a property of an mm that is only rarely changed, and it doesn't really belong as part of the tlb_gen mechanism.  And, critically, it's not worth the effort and complexity to try to optimize LAM changes when we have a lazy CPU (just like PCE and LDT) (whereas TLB flushes are performance critical and are absolutely worth optimizing).
> 
> So...
> 
> >
> > Fix this by making sure we write a new CR3 if LAM is not up-to-date. No
> > problems were observed in practice, this was found by code inspection.
> 
> I think it should be fixed with a much bigger hammer: explicit IPIs.  Just don't ever let it get out of date, like install_ldt().

I like this, and I think earlier versions of the code did this. I think
the code now assumes it's fine to not send an IPI since only
single-threaded processes can enable LAM, but this means we have to
handle kthreads switching to user threads with outdated LAMs (what this
patch is trying to do).

I also think there is currently an assumption that it's fine for
kthreads to run with an incorrect LAM, which is mostly fine, but the IPI
also drops that assumption.

> 
> >
> > Not that it is possible that mm->context.lam_cr3_mask changes throughout
> > switch_mm_irqs_off(). But since LAM can only be enabled by a
> > single-threaded process on its own behalf, in that case we cannot be
> > switching to a user thread in that same process, we can only be
> > switching to another kthread using the borrowed mm or a different user
> > process, which should be fine.
> 
> The thought process is even simpler with the IPI: it *can* change while switching, but it will resynchronize immediately once IRQs turn back on.  And whoever changes it will *synchronize* with us, which would otherwise require extremely complex logic to get right.
> 
> And...
> 
> > -		if (!was_lazy)
> > -			return;
> > +		if (was_lazy) {
> > +			/*
> > +			 * Read the tlb_gen to check whether a flush is needed.
> > +			 * If the TLB is up to date, just use it.  The barrier
> > +			 * synchronizes with the tlb_gen increment in the TLB
> > +			 * shootdown code.
> > +			 */
> > +			smp_mb();
> 
> This is actually rather expensive -- from old memory, we're talking maybe 20 cycles here, but this path is *very* hot and we try fairly hard to make it be fast.  If we get the happy PCID path, it's maybe 100-200 cycles, so this is like a 10% regression.  Ouch.

This is not newly introduced by this patch. I merely refactored this
code (reversed the if conditions). I think if we keep the current
approach I should move this refactoring to a separate patch to make
things clearer.

> 
> And you can delete all of this if you accept my suggestion.

I like it very much. The problem now is, as I told Dave, I realized I
cannot do any testing beyond compilation due to lack of hardware. I am
happy to send a next version if this is acceptable or if someone else
can test.