On Mon, Sep 20, 2021, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>
>
> There is no need to copy the whole memslot data after releasing
> slots_arch_lock for a moment to install temporary memslots copy in
> kvm_set_memslot() since this lock only protects the arch field of each
> memslot.
>
> Just resync this particular field after reacquiring slots_arch_lock.
I assume this needed to avoid having a mess when introducing the r-b tree? If so,
please call that out. Iterating over the slots might actually be slower than the
full memcpy, i.e. as a standalone patch this may or may not be make sense.
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
> ---
> virt/kvm/kvm_main.c | 17 ++++++++++++-----
> 1 file changed, 12 insertions(+), 5 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 348fae880189..48d182840060 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1482,6 +1482,15 @@ static void kvm_copy_memslots(struct kvm_memslots *to,
> memcpy(to, from, kvm_memslots_size(from->used_slots));
> }
>
> +static void kvm_copy_memslots_arch(struct kvm_memslots *to,
> + struct kvm_memslots *from)
> +{
> + int i;
> +
> + for (i = 0; i < from->used_slots; i++)
> + to->memslots[i].arch = from->memslots[i].arch;
This should probably be a memcpy(), I don't know what all shenanigans the compiler
can throw at us if it gets to copy a struct by value.
> +}
> +
> /*
> * Note, at a minimum, the current number of used slots must be allocated, even
> * when deleting a memslot, as we need a complete duplicate of the memslots for
There's an out-of-sight comment that's now stale, can you revert to the
pre-slots_arch_lock comment?
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 48d182840060..ef3345428047 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1555,9 +1555,10 @@ static int kvm_set_memslot(struct kvm *kvm,
slot->flags |= KVM_MEMSLOT_INVALID;
/*
- * We can re-use the memory from the old memslots.
- * It will be overwritten with a copy of the new memslots
- * after reacquiring the slots_arch_lock below.
+ * We can re-use the old memslots, the only difference from the
+ * newly installed memslots is the invalid flag, which will get
+ * dropped by update_memslots anyway. We'll also revert to the
+ * old memslots if preparing the new memory region fails.
*/
slots = install_new_memslots(kvm, as_id, slots);
> @@ -1567,10 +1576,10 @@ static int kvm_set_memslot(struct kvm *kvm,
> /*
> * The arch-specific fields of the memslots could have changed
> * between releasing the slots_arch_lock in
> - * install_new_memslots and here, so get a fresh copy of the
> - * slots.
> + * install_new_memslots and here, so get a fresh copy of these
> + * fields.
> */
> - kvm_copy_memslots(slots, __kvm_memslots(kvm, as_id));
> + kvm_copy_memslots_arch(slots, __kvm_memslots(kvm, as_id));
> }
>
> r = kvm_arch_prepare_memory_region(kvm, old, new, mem, change);
> @@ -1587,8 +1596,6 @@ static int kvm_set_memslot(struct kvm *kvm,
>
> out_slots:
> if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) {
> - slot = id_to_memslot(slots, old->id);
> - slot->flags &= ~KVM_MEMSLOT_INVALID;
> slots = install_new_memslots(kvm, as_id, slots);
> } else {
The braces can be dropped since both branches are now single lines.
> mutex_unlock(&kvm->slots_arch_lock);
