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[v9,15/17] KVM: Introduce the KVM capability KVM_CAP_GMEM_SHARED_MEM

Message ID 20250513163438.3942405-16-tabba@google.com
State New
Headers show
Series KVM: Mapping guest_memfd backed memory at the host for software protected VMs | expand

Commit Message

Fuad Tabba May 13, 2025, 4:34 p.m. UTC 
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@google.com>
---
 Documentation/virt/kvm/api.rst | 18 ++++++++++++++++++
 include/uapi/linux/kvm.h       |  1 +
 virt/kvm/kvm_main.c            |  4 ++++
 3 files changed, 23 insertions(+)
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Patch

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.
+
+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.
 
 4.143 KVM_PRE_FAULT_MEMORY
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 9857022a0f0c..4cc824a3a7c9 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -930,6 +930,7 @@  struct kvm_enable_cap {
 #define KVM_CAP_X86_APIC_BUS_CYCLES_NS 237
 #define KVM_CAP_X86_GUEST_MODE 238
 #define KVM_CAP_ARM_WRITABLE_IMP_ID_REGS 239
+#define KVM_CAP_GMEM_SHARED_MEM 240
 
 struct kvm_irq_routing_irqchip {
 	__u32 irqchip;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 6261d8638cd2..6c75f933bfbe 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4840,6 +4840,10 @@  static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
 #ifdef CONFIG_KVM_GMEM
 	case KVM_CAP_GUEST_MEMFD:
 		return !kvm || kvm_arch_supports_gmem(kvm);
+#endif
+#ifdef CONFIG_KVM_GMEM_SHARED_MEM
+	case KVM_CAP_GMEM_SHARED_MEM:
+		return !kvm || kvm_arch_vm_supports_gmem_shared_mem(kvm);
 #endif
 	default:
 		break;