Re: [PATCH v3 7/8] KVM: x86/mmu: Protect rmaps independently with SRCU

[Sean Christopherson <seanjc@google.com>]

On Fri, May 07, 2021, Paolo Bonzini wrote:
> On 06/05/21 20:42, Ben Gardon wrote:
> > In preparation for lazily allocating the rmaps when the TDP MMU is in
> > use, protect the rmaps with SRCU. Unfortunately, this requires
> > propagating a pointer to struct kvm around to several functions.
> 
> Thinking more about it, this is not needed because all reads of the rmap
> array are guarded by the load-acquire of kvm->arch.memslots_have_rmaps.
> That is, the pattern is always
> 
> 	if (!load-acquire(memslot_have_rmaps))
> 		return;
> 	... = __gfn_to_rmap(...)
> 
> 				slots->arch.rmap[x] = ...
> 				store-release(memslot_have_rmaps, true)
> 
> where the load-acquire/store-release have the same role that
> srcu_dereference/rcu_assign_pointer had before this patch.
> 
> We also know that any read that misses the check has the potential for a
> NULL pointer dereference, so it *has* to be like that.
> 
> That said, srcu_dereference has zero cost unless debugging options are
> enabled, and it *is* true that the rmap can disappear if kvm->srcu is not
> held, so I lean towards keeping this change and just changing the commit
> message like this:
> 
> ---------
> Currently, rmaps are always allocated and published together with a new
> memslot, so the srcu_dereference for the memslots array already ensures that
> the memory pointed to by slots->arch.rmap is zero at the time
> slots->arch.rmap.  However, they still need to be accessed in an SRCU
> read-side critical section, as the whole memslot can be deleted outside
> SRCU.
> --------

I disagree, sprinkling random and unnecessary __rcu/SRCU annotations does more
harm than good.  Adding the unnecessary tag could be quite misleading as it
would imply the rmap pointers can _change_ independent of the memslots.

Similary, adding rcu_assign_pointer() in alloc_memslot_rmap() implies that its
safe to access the rmap after its pointer is assigned, and that's simply not
true since an rmap array can be freed if rmap allocation for a different memslot
fails.  Accessing the rmap is safe if and only if all rmaps are allocated, i.e.
if arch.memslots_have_rmaps is true, as you pointed out.

Furthermore, to actually gain any protection from SRCU, there would have to be
an synchronize_srcu() call after assigning the pointers, and that _does_ have an
associated.  Not to mention that to truly respect the __rcu annotation, deleting
the rmaps would also have to be done "independently" with the correct
rcu_assign_pointer() and synchronize_srcu() logic.