On Tue, Aug 13, 2024, Lai Jiangshan wrote:
> On Mon, Aug 12, 2024 at 11:22 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> >
> > Oh yeah, duh, re-read after PAUSE, not before.
> >
> > Definitely holler if you have any alternative ideas for walking rmaps
> > without taking mmu_lock, I guarantee you've spent more time than me
> > thinking about the shadow MMU :-)
>
> We use the same bit and the same way for the rmap lock.
>
> We just use bit_spin_lock() and the optimization for empty rmap_head is
> handled out of kvm_rmap_lock().
Hmm, I'm leaning towards keeping the custom locking. There are a handful of
benefits, none of which are all that meaningful on their own, but do add up.
- Callers don't need to manually check for an empty rmap_head.
- Can avoid the redundant preempt_{disable,enable}() entirely in the common case
of being called while mmu_lock is held.
- Handles the (likely super rare) edge case where a read-only walker encounters
an rmap that was just emptied (rmap_head->val goes to zero after the initial
check to elide the lock).
- Avoids an atomic when releasing the lock, and any extra instructions entirely
for writers since they always write the full rmap_head->val when releasing.
> bit_spin_lock() has the most-needed preempt_disable(). I'm not sure if the
> new kvm_rmap_age_gfn_range_lockless() is called in a preempt disabled region.
Oof, it doesn't. Disabling IRQs crossed my mind, but I completely forgot about
preemption.
Thanks much!
