Re: [PATCH 3/8] kvm: Detect guest-memfd flags supported

[Xiaoyao Li]

On 10/24/2025 2:59 AM, Peter Xu wrote:
Detect supported guest-memfd flags by the current kernel, and reject
creations of guest-memfd using invalid flags.  When the cap isn't
available, then no flag is supported.
Signed-off-by: Peter Xu <pet...@redhat.com>
---
  accel/kvm/kvm-all.c | 12 ++++++++++++
  1 file changed, 12 insertions(+)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 1425dfd8b3..48a8f6424f 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -108,6 +108,7 @@ static int kvm_sstep_flags;
  static bool kvm_immediate_exit;
  static uint64_t kvm_supported_memory_attributes;
  static bool kvm_guest_memfd_supported;
+static uint64_t kvm_guest_memfd_flags_supported;
  static hwaddr kvm_max_slot_size = ~0;
  
  static const KVMCapabilityInfo kvm_required_capabilites[] = {
@@ -2781,6 +2782,11 @@ static int kvm_init(AccelState *as, MachineState *ms)
      kvm_supported_memory_attributes = kvm_vm_check_extension(s, 
KVM_CAP_MEMORY_ATTRIBUTES);
      kvm_guest_memfd_supported = kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD) 
&&
          kvm_vm_check_extension(s, KVM_CAP_USER_MEMORY2);
+    ret = kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD_FLAGS);
+    if (ret > 0)
+        kvm_guest_memfd_flags_supported = (uint64_t)ret;
+    else
+        kvm_guest_memfd_flags_supported = 0;

Nit:
1. QEMU's coding style always requires curly braces.
2. is the (uint64_t) necessary?
3. can we name it "kvm_supported_guest_memfd_flags" to make it 
consistent with "kvm_supported_memory_attributes"?

so how about

kvm_supported_guest_memfd_flags = kvm_vm_check_extension(s, 
KVM_CAP_GUEST_MEMFD_FLAGS);
    if (kvm_supported_guest_memfd_flags < 0) {
        kvm_supported_guest_memfd_flags = 0;
    }

      kvm_pre_fault_memory_supported = kvm_vm_check_extension(s, 
KVM_CAP_PRE_FAULT_MEMORY);
  
      if (s->kernel_irqchip_split == ON_OFF_AUTO_AUTO) {
@@ -4486,6 +4492,12 @@ int kvm_create_guest_memfd(uint64_t size, uint64_t 
flags, Error **errp)
          return -1;
      }
  
+    if (flags & ~kvm_guest_memfd_flags_supported) {
+        error_setg(errp, "KVM does not support guest-memfd flag: 0x%"PRIx64,
+                   flags & ~kvm_guest_memfd_flags_supported);
+        return -1;
+    }
+
      fd = kvm_vm_ioctl(kvm_state, KVM_CREATE_GUEST_MEMFD, &guest_memfd);
      if (fd < 0) {
          error_setg_errno(errp, errno, "Error creating KVM guest_memfd");