KVM: guest_memfd: Track guest_memfd mmap support in memslot

Add a new internal flag, KVM_MEMSLOT_GMEM_ONLY, to the top half of
memslot->flags (which makes it strictly for KVM's internal use). This
flag tracks when a guest_memfd-backed memory slot supports host
userspace mmap operations, which implies that all memory, not just
private memory for CoCo VMs, is consumed through guest_memfd: "gmem
only".

This optimization avoids repeatedly checking the underlying guest_memfd
file for mmap support, which would otherwise require taking and
releasing a reference on the file for each check. By caching this
information directly in the memslot, we reduce overhead and simplify the
logic involved in handling guest_memfd-backed pages for host mappings.

Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Shivank Garg <shivankg@amd.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Acked-by: David Hildenbrand <david@redhat.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250729225455.670324-12-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 26bad600f9fa32714b505de41079fa0067d26fff..8b47891adca18779dd277db6f90fdeaafe2a52f3 100644 (file)
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -54,7 +54,8 @@
  * used in kvm, other bits are visible for userspace which are defined in
  * include/uapi/linux/kvm.h.
  */
-#define KVM_MEMSLOT_INVALID    (1UL << 16)
+#define KVM_MEMSLOT_INVALID                    (1UL << 16)
+#define KVM_MEMSLOT_GMEM_ONLY                  (1UL << 17)
 
 /*
  * Bit 63 of the memslot generation number is an "update in-progress flag",
@@ -2490,6 +2491,14 @@ static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
                vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_PRIVATE;
 }
 
+static inline bool kvm_memslot_is_gmem_only(const struct kvm_memory_slot *slot)
+{
+       if (!IS_ENABLED(CONFIG_KVM_GUEST_MEMFD))
+               return false;
+
+       return slot->flags & KVM_MEMSLOT_GMEM_ONLY;
+}
+
 #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
 static inline unsigned long kvm_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
 {

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 67e7cd7210ef63884921181a3123cbdf2b7144ca..d5b445548af428c91f0149b9144b592c3fe13ebd 100644 (file)
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -578,6 +578,8 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
         */
        WRITE_ONCE(slot->gmem.file, file);
        slot->gmem.pgoff = start;
+       if (kvm_gmem_supports_mmap(inode))
+               slot->flags |= KVM_MEMSLOT_GMEM_ONLY;
 
        xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL);
        filemap_invalidate_unlock(inode->i_mapping);