Hi Fuad,
On 5/14/25 2:34 AM, Fuad Tabba wrote:
This patch introduces the KVM capability KVM_CAP_GMEM_SHARED_MEM, which
indicates that guest_memfd supports shared memory (when enabled by the
flag). This support is limited to certain VM types, determined per
architecture.
This patch also updates the KVM documentation with details on the new
capability, flag, and other information about support for shared memory
in guest_memfd.
Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
---
Documentation/virt/kvm/api.rst | 18 ++++++++++++++++++
include/uapi/linux/kvm.h | 1 +
virt/kvm/kvm_main.c | 4 ++++
3 files changed, 23 insertions(+)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 47c7c3f92314..86f74ce7f12a 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6390,6 +6390,24 @@ most one mapping per page, i.e. binding multiple memory regions to a single
guest_memfd range is not allowed (any number of memory regions can be bound to
a single guest_memfd file, but the bound ranges must not overlap).
+When the capability KVM_CAP_GMEM_SHARED_MEM is supported, the 'flags' field
+supports GUEST_MEMFD_FLAG_SUPPORT_SHARED. Setting this flag on guest_memfd
+creation enables mmap() and faulting of guest_memfd memory to host userspace.
+
+When the KVM MMU performs a PFN lookup to service a guest fault and the backing
+guest_memfd has the GUEST_MEMFD_FLAG_SUPPORT_SHARED set, then the fault will
+always be consumed from guest_memfd, regardless of whether it is a shared or a
+private fault.
+
+For these memslots, userspace_addr is checked to be the mmap()-ed view of the
+same range specified using gmem.pgoff. Other accesses by KVM, e.g., instruction
+emulation, go via slot->userspace_addr. The slot->userspace_addr field can be
+set to 0 to skip this check, which indicates that KVM would not access memory
+belonging to the slot via its userspace_addr.
+
This paragraph needs to be removed if PATCH[08/17] is going to be dropped.
[PATCH v9 08/17] KVM: guest_memfd: Check that userspace_addr and fd+offset refer to same range
+The use of GUEST_MEMFD_FLAG_SUPPORT_SHARED will not be allowed for CoCo VMs.
+This is validated when the guest_memfd instance is bound to the VM.
+
See KVM_SET_USER_MEMORY_REGION2 for additional details.
[...]
Thanks,
Gavin
