On 12.11.2021 01:51, Sean Christopherson wrote:
On Fri, Nov 12, 2021, Maciej S. Szmigiero wrote:
On 04.11.2021 01:25, Sean Christopherson wrote:
- /*
- * Remove the old memslot from the hash list and interval tree, copying
- * the node data would corrupt the structures.
- */
+ int as_id = kvm_memslots_get_as_id(old, new);
+ struct kvm_memslots *slots = kvm_get_inactive_memslots(kvm, as_id);
+ int idx = slots->node_idx;
+
if (old) {
- hash_del(&old->id_node);
- interval_tree_remove(&old->hva_node, &slots->hva_tree);
+ hash_del(&old->id_node[idx]);
+ interval_tree_remove(&old->hva_node[idx], &slots->hva_tree);
- if (!new)
+ if ((long)old == atomic_long_read(&slots->last_used_slot))
+ atomic_long_set(&slots->last_used_slot, (long)new);
Open-coding cmpxchg() is way less readable than a direct call.
Doh, I meant to call this out and/or add a comment.
My objection to cmpxchg() is that it implies atomicity is required (the kernel's
version adds the lock), which is very much not the case. So this isn't strictly
an open-coded version of cmpxchg().
The open-coded version also compiles on x86 to multiple instructions with
a branch, instead of just a single instruction.
Yeah. The lock can't be contended, so that part of cmpxchg is a non-issue. But
that's also why I don't love using cmpxchg.
I don't have a strong preference, I just got briefly confused by the atomicity part.
We can simply add a comment there to explain that the atomicity isn't actually
strictly required here - will do that.
+static void kvm_invalidate_memslot(struct kvm *kvm,
+ struct kvm_memory_slot *old,
+ struct kvm_memory_slot *working_slot)
+{
+ /*
+ * Mark the current slot INVALID. As with all memslot modifications,
+ * this must be done on an unreachable slot to avoid modifying the
+ * current slot in the active tree.
+ */
+ kvm_copy_memslot(working_slot, old);
+ working_slot->flags |= KVM_MEMSLOT_INVALID;
+ kvm_replace_memslot(kvm, old, working_slot);
+
+ /*
+ * Activate the slot that is now marked INVALID, but don't propagate
+ * the slot to the now inactive slots. The slot is either going to be
+ * deleted or recreated as a new slot.
+ */
+ kvm_swap_active_memslots(kvm, old->as_id);
+
+ /*
+ * From this point no new shadow pages pointing to a deleted, or moved,
+ * memslot will be created. Validation of sp->gfn happens in:
+ * - gfn_to_hva (kvm_read_guest, gfn_to_pfn)
+ * - kvm_is_visible_gfn (mmu_check_root)
+ */
+ kvm_arch_flush_shadow_memslot(kvm, old);
This should flush the currently active slot (that is, "working_slot",
not "old") to not introduce a behavior change with respect to the existing
code.
That's also what the previous version of this patch set did.
Eww. I would much prefer to "fix" the existing code in a prep patch. It shouldn't
matter, but arch code really should not get passed an INVALID slot.
I will add a separate patch that switches that kvm_arch_flush_shadow_memslot()
call to use a valid (old) memslot instead.
It is actually simpler to do it *after* the main patch series to not add
more dead code that next patches remove anyway.
Thanks,
Maciej
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