Re: [PATCH 3/3] accel/kvm: Set mem.guest_memfd_offset only when guest_memfd is valid

[Philippe Mathieu-Daudé]

On 23/7/25 09:09, Xiaoyao Li wrote:

Current QEMU unconditionally sets the mmem.guest_memfd_offset in
kvm_set_user_memory_region(), which leads to the trace of kvm_set_user_memory
look as:

kvm_set_user_memory AddrSpace#0 Slot#4 flags=0x2 gpa=0xe0000 size=0x20000 
ua=0x7f5840de0000 guest_memfd=-1 guest_memfd_offset=0x3e0000 ret=0

It's confusing that the guest_memfd_offset has a non-zero value while
the guest_memfd is invalid (-1).

Change to only set guest_memfd_offset when guest_memfd is valid.

Signed-off-by: Xiaoyao Li <xiaoyao...@intel.com>
---
  accel/kvm/kvm-all.c | 4 +++-
  1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 4f4c30fc84b2..2b6fbf58a127 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -367,7 +367,9 @@ static int kvm_set_user_memory_region(KVMMemoryListener 
*kml, KVMSlot *slot, boo
      mem.userspace_addr = (unsigned long)slot->ram;
      mem.flags = slot->flags;
      mem.guest_memfd = slot->guest_memfd;
-    mem.guest_memfd_offset = slot->guest_memfd_offset;
+    if (slot->guest_memfd >= 0) {
+        mem.guest_memfd_offset = slot->guest_memfd_offset;
+    }

IMHO the issue is in when the KVMSlot is filled in kvm_set_phys_mem():

-- >8 --
diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 890d5ea9f86..631f14b996a 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -1595,7 +1595,8 @@ static void kvm_set_phys_mem(KVMMemoryListener *kml,
         mem->ram = ram;
         mem->flags = kvm_mem_flags(mr);
         mem->guest_memfd = mr->ram_block->guest_memfd;
-        mem->guest_memfd_offset = (uint8_t*)ram - mr->ram_block->host;
+        mem->guest_memfd_offset = mr->ram_block->guest_memfd
+                                ? (uint8_t*)ram - mr->ram_block->host : 0;

         kvm_slot_init_dirty_bitmap(mem);
         err = kvm_set_user_memory_region(kml, mem, true);
---