Re: [PATCH 19/22] KVM: x86/mmu: Add infrastructure to allow walking rmaps outside of mmu_lock

[Lai Jiangshan <jiangshanlai@xxxxxxxxx>]

On Sat, Aug 10, 2024 at 3:49 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:

> +
> +static unsigned long kvm_rmap_lock(struct kvm_rmap_head *rmap_head)
> +{
> +       unsigned long old_val, new_val;
> +
> +       old_val = READ_ONCE(rmap_head->val);
> +       if (!old_val)
> +               return 0;
> +
> +       do {
> +               /*
> +                * If the rmap is locked, wait for it to be unlocked before
> +                * trying acquire the lock, e.g. to bounce the cache line.
> +                */
> +               while (old_val & KVM_RMAP_LOCKED) {
> +                       old_val = READ_ONCE(rmap_head->val);
> +                       cpu_relax();

The sequence of these two lines of code can be improved.

> +               }
> +
> +               /*
> +                * Recheck for an empty rmap, it may have been purged by the
> +                * task that held the lock.
> +                */
> +               if (!old_val)
> +                       return 0;
> +
> +               new_val = old_val | KVM_RMAP_LOCKED;
> +       } while (!try_cmpxchg(&rmap_head->val, &old_val, new_val));
> +
> +       /* Return the old value, i.e. _without_ the LOCKED bit set. */
> +       return old_val;
> +}