On 7/16/2025 4:21 PM, Fuad Tabba wrote:
Hi Xiaoyao,
On Wed, 16 Jul 2025 at 07:11, Xiaoyao Li <xiaoyao.li@xxxxxxxxx> wrote:
On 7/15/2025 5:33 PM, Fuad Tabba wrote:
Add a new internal flag, KVM_MEMSLOT_GMEM_ONLY, to the top half of
memslot->flags. This flag tracks when a guest_memfd-backed memory slot
supports host userspace mmap operations. It's strictly for KVM's
internal use.
I would expect some clarification of why naming it with
KVM_MEMSLOT_GMEM_ONLY, not something like KVM_MEMSLOT_GMEM_MMAP_ENABLED
There was a patch to check the userspace_addr of the memslot refers to
the same memory as guest memfd[1], but that patch was dropped. Without
the background that when guest memfd is mmapable, userspace doesn't need
to provide separate memory via userspace_addr, it's hard to understand
and accept the name of GMEM_ONLY.
The commit message could have clarified this a bit more. Regarding the
rationale for the naming, there have been various threads and live
discussions in the biweekly guest_memfd meeting . Instead of rehashing
the discussion here, I can refer you to a couple [1, 2].
I don't object the name. Just want the clarification in commit message
and even better add comment in code. That will be truly helpful for
future readers.
[1] https://docs.google.com/document/d/1M6766BzdY1Lhk7LiR5IqVR8B8mG3cr-cxTxOrAosPOk/edit?tab=t.0#heading=h.a15es1buok51
[2] https://lore.kernel.org/all/aFwChljXL5QJYLM_@xxxxxxxxxx/
Thanks,
/fuad
[1] https://lore.kernel.org/all/20250513163438.3942405-9-tabba@xxxxxxxxxx/
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@xxxxxxxxxx>
Reviewed-by: Shivank Garg <shivankg@xxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
---
include/linux/kvm_host.h | 11 ++++++++++-
virt/kvm/guest_memfd.c | 2 ++
2 files changed, 12 insertions(+), 1 deletion(-)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 9ac21985f3b5..d2218ec57ceb 100644
--- 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",
@@ -2536,6 +2537,14 @@ 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_GMEM_SUPPORTS_MMAP))
+ 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 07a4b165471d..2b00f8796a15 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -592,6 +592,8 @@ 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);
