[PATCH v2 0/3] kvm: guest_memfd enhancement and fix for KVM_SET_USER_MEMORY_REGION2

[PATCH v2 0/3] kvm: guest_memfd enhancement and fix for KVM_SET_USER_MEMORY_REGION2

[Xiaoyao Li]

Patch 1 changes to check guest memfd CAP on VM instance.

Patch 2 is a fix for KVM_SET_USER_MEMORY_REGION2.

Patch 3 is an enhancement for guest_memfd_offset.

---
Changes in v2:
- Collect R-B tags;
- Use {} to zero out the struct instead of memset() in patch2;
- Initialize guest_memfd_offset at the original source in patch3;

Xiaoyao Li (3):
  accel/kvm: Switch to check KVM_CAP_GUEST_MEMFD and
    KVM_CAP_USER_MEMORY2 on VM
  accel/kvm: Zero out mem explicitly in kvm_set_user_memory_region()
  accel/kvm: Set guest_memfd_offset to non-zero value only when
    guest_memfd is valid

 accel/kvm/kvm-all.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

-- 
2.43.0

[PATCH v2 1/3] accel/kvm: Switch to check KVM_CAP_GUEST_MEMFD and KVM_CAP_USER_MEMORY2 on VM

[Xiaoyao Li]

It returns more accruate result on checking KVM_CAP_GUEST_MEMFD and
KVM_CAP_USER_MEMORY2 on VM instance instead of on KVM platform.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
---
 accel/kvm/kvm-all.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 890d5ea9f865..14d47246ca63 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -2776,8 +2776,8 @@ 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_check_extension(s, KVM_CAP_GUEST_MEMFD) &&
-        kvm_check_extension(s, KVM_CAP_USER_MEMORY2) &&
+        kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD) &&
+        kvm_vm_check_extension(s, KVM_CAP_USER_MEMORY2) &&
         (kvm_supported_memory_attributes & KVM_MEMORY_ATTRIBUTE_PRIVATE);
     kvm_pre_fault_memory_supported = kvm_vm_check_extension(s, KVM_CAP_PRE_FAULT_MEMORY);
 
-- 
2.43.0

[PATCH v2 2/3] accel/kvm: Zero out mem explicitly in kvm_set_user_memory_region()

[Xiaoyao Li]

Zero out the entire mem explicitly before it's used, to ensure the unused
feilds (pad1, pad2) are all zeros. Otherwise, it might cause problem when
the pad fields are extended by future KVM.

Fixes: ce5a983233b4 ("kvm: Enable KVM_SET_USER_MEMORY_REGION2 for memslot")
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
changes in v2:
- using {} instead of memset(); (suggested by Philippe)
---
 accel/kvm/kvm-all.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 14d47246ca63..90f3b177a1ff 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -358,7 +358,7 @@ int kvm_physical_memory_addr_from_host(KVMState *s, void *ram,
 static int kvm_set_user_memory_region(KVMMemoryListener *kml, KVMSlot *slot, bool new)
 {
     KVMState *s = kvm_state;
-    struct kvm_userspace_memory_region2 mem;
+    struct kvm_userspace_memory_region2 mem = {};
     int ret;
 
     mem.slot = slot->slot | (kml->as_id << 16);
-- 
2.43.0

[PATCH v2 3/3] accel/kvm: Set guest_memfd_offset to non-zero value only when guest_memfd is valid

[Xiaoyao Li]

Current QEMU unconditionally sets the guest_memfd_offset of KVMSlot in
kvm_set_phys_mem(), which leads to the trace of kvm_set_user_memory looks:

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 and
leave it as 0 when no valid guest_memfd.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
---
Changes in v2:
- initialize guest_memfd_offset in kvm_set_phys_mem(); (Phillippe)
---
 accel/kvm/kvm-all.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 90f3b177a1ff..cc694fe3c3a9 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 = mem->guest_memfd >= 0 ?
+                                  (uint8_t*)ram - mr->ram_block->host : 0;
 
         kvm_slot_init_dirty_bitmap(mem);
         err = kvm_set_user_memory_region(kml, mem, true);
-- 
2.43.0

Re: [PATCH v2 0/3] kvm: guest_memfd enhancement and fix for KVM_SET_USER_MEMORY_REGION2

[Xiaoyao Li]

On 7/28/2025 7:57 PM, Xiaoyao Li wrote:
> Patch 1 changes to check guest memfd CAP on VM instance.
> 
> Patch 2 is a fix for KVM_SET_USER_MEMORY_REGION2.
> 
> Patch 3 is an enhancement for guest_memfd_offset.

Gentle ping.

> ---
> Changes in v2:
> - Collect R-B tags;
> - Use {} to zero out the struct instead of memset() in patch2;
> - Initialize guest_memfd_offset at the original source in patch3;
> 
> Xiaoyao Li (3):
>    accel/kvm: Switch to check KVM_CAP_GUEST_MEMFD and
>      KVM_CAP_USER_MEMORY2 on VM
>    accel/kvm: Zero out mem explicitly in kvm_set_user_memory_region()
>    accel/kvm: Set guest_memfd_offset to non-zero value only when
>      guest_memfd is valid
> 
>   accel/kvm/kvm-all.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
> 

Re: [PATCH v2 0/3] kvm: guest_memfd enhancement and fix for KVM_SET_USER_MEMORY_REGION2

[Paolo Bonzini]

Queued, thanks.

Paolo