[PATCH v3] KVM: Skip gfn-based MMU notifier invalidations for guest_memfd-only memslots

[Sean Christopherson <seanjc@google.com>]

From: Alexandru Elisei <alexandru.elisei@arm.com>

Don't invalidate gmem-only memslots when responding to MMU notifier
invalidations, as the entire point of gmem-only memslots is so that KVM
doesn't need a VMA to install mappings into KVM's MMU.  Specifically,
page faults are resolved by going straight to guest_memfd, using the gfn
and the associated memslot's gmem.file.  I.e. whether or not userspace
happens to have valid mappings to the same gmem instance is irrelevant,
as are any invalidations for those mappings.

Note!  The exception is KVM's gfn_to_pfn_cache implementation which isn't
yet wired up to pull pfns directly from guest_memfd, i.e. still relies on
userspace mappings/VMAs and thus need to be invalidated, e.g. to ensure KVM
doesn't retain a kernel mapping+pointer to PRIVATE memory.  However, as of
commit da260bcb56fe ("KVM: pfncache: use a dedicated invalidation sequence
for cache refresh"), GPC invalidation no longer relies on memslot lookups,
because GPCs also support HVA-based mappings.  I.e. KVM *must* be able to
detect and handle relevant MMU notifier invalidations even if there is *no*
memslot whatsoever.

In addition to the obvious performance benefits, avoiding unnecessary and
spurious SPTE zaps and TLB flushes will allow KVM to support features like
ARM's Statistical Profiling Extension (SPE)[1] and x86's Secure AVIC[2],
both of which rely on the pages *never* being unmapped from the stage-2
page tables.

Link: https://lore.kernel.org/all/20260903160623.315525-1-alexandru.elisei@arm.com [1]
Link: https://lore.kernel.org/all/cover.1783490022.git.naveen@kernel.org [2]
Cc: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: Naveen N Rao (AMD) <naveen@kernel.org>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
[sean: rewrite changelog to capture the new gpc behavior]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---

v3: v1, but with a different changelog.

v2: https://lore.kernel.org/all/20260714160411.302386-1-alexandru.elisei@arm.com

v1: https://lore.kernel.org/all/20260615155244.183044-1-alexandru.elisei@arm.com

 virt/kvm/kvm_main.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index cf4cb586f140..599dea22afb8 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -601,6 +601,10 @@ static __always_inline kvm_mn_ret_t kvm_handle_hva_range(struct kvm *kvm,
 			unsigned long hva_start, hva_end;
 
 			slot = container_of(node, struct kvm_memory_slot, hva_node[slots->node_idx]);
+
+			if (kvm_slot_has_gmem(slot) && kvm_memslot_is_gmem_only(slot))
+				continue;
+
 			hva_start = max_t(unsigned long, range->start, slot->userspace_addr);
 			hva_end = min_t(unsigned long, range->end,
 					slot->userspace_addr + (slot->npages << PAGE_SHIFT));

base-commit: d21eef2084a9af87ac2a783bc7c2b175ba114e4e
-- 
2.56.1.703.gf848ed807c-goog