[PATCH v13 14/20] KVM: x86: Enable guest_memfd mmap for default VM type

[Fuad Tabba <tabba@google.com>]

Enable host userspace mmap support for guest_memfd-backed memory when
running KVM with the KVM_X86_DEFAULT_VM type:

* Define kvm_arch_supports_gmem_mmap() for KVM_X86_DEFAULT_VM: Introduce
  the architecture-specific kvm_arch_supports_gmem_mmap() macro,
  specifically enabling mmap support for KVM_X86_DEFAULT_VM instances.
  This macro, gated by CONFIG_KVM_GMEM_SUPPORTS_MMAP, ensures that only
  the default VM type can leverage guest_memfd mmap functionality on
  x86. This explicit enablement prevents CoCo VMs, which use guest_memfd
  primarily for private memory and rely on hardware-enforced privacy,
  from accidentally exposing guest memory via host userspace mappings.

* Select CONFIG_KVM_GMEM_SUPPORTS_MMAP in KVM_X86: Enable the
  CONFIG_KVM_GMEM_SUPPORTS_MMAP Kconfig option when KVM_X86 is selected.
  This ensures that the necessary code for guest_memfd mmap support
  (introduced earlier) is compiled into the kernel for x86. This Kconfig
  option acts as a system-wide gate for the guest_memfd mmap capability.
  It implicitly enables CONFIG_KVM_GMEM, making guest_memfd available,
  and then layers the mmap capability on top specifically for the
  default VM.

These changes make guest_memfd a more versatile memory backing for
standard KVM guests, allowing VMMs to use a unified guest_memfd model
for both private (CoCo) and non-private (default) VMs. This is a
prerequisite for use cases such as running Firecracker guests entirely
backed by guest_memfd and implementing direct map removal for non-CoCo
VMs.

Co-developed-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 arch/x86/include/asm/kvm_host.h | 9 +++++++++
 arch/x86/kvm/Kconfig            | 1 +
 arch/x86/kvm/x86.c              | 3 ++-
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 4c764faa12f3..4c89feaa1910 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2273,9 +2273,18 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
 #ifdef CONFIG_KVM_GMEM
 #define kvm_arch_has_private_mem(kvm) ((kvm)->arch.has_private_mem)
 #define kvm_arch_supports_gmem(kvm)  ((kvm)->arch.supports_gmem)
+
+/*
+ * CoCo VMs with hardware support that use guest_memfd only for backing private
+ * memory, e.g., TDX, cannot use guest_memfd with userspace mapping enabled.
+ */
+#define kvm_arch_supports_gmem_mmap(kvm)		\
+	(IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP) &&	\
+	 (kvm)->arch.vm_type == KVM_X86_DEFAULT_VM)
 #else
 #define kvm_arch_has_private_mem(kvm) false
 #define kvm_arch_supports_gmem(kvm) false
+#define kvm_arch_supports_gmem_mmap(kvm) false
 #endif
 
 #define kvm_arch_has_readonly_mem(kvm) (!(kvm)->arch.has_protected_state)
diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index df1fdbb4024b..239637b663dc 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -47,6 +47,7 @@ config KVM_X86
 	select KVM_GENERIC_HARDWARE_ENABLING
 	select KVM_GENERIC_PRE_FAULT_MEMORY
 	select KVM_GENERIC_GMEM_POPULATE if KVM_SW_PROTECTED_VM
+	select KVM_GMEM_SUPPORTS_MMAP
 	select KVM_WERROR if WERROR
 
 config KVM
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b34236029383..17c655e5716e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12779,7 +12779,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 
 	kvm->arch.vm_type = type;
 	kvm->arch.has_private_mem = (type == KVM_X86_SW_PROTECTED_VM);
-	kvm->arch.supports_gmem = (type == KVM_X86_SW_PROTECTED_VM);
+	kvm->arch.supports_gmem =
+		type == KVM_X86_DEFAULT_VM || type == KVM_X86_SW_PROTECTED_VM;
 	/* Decided by the vendor code for other VM types.  */
 	kvm->arch.pre_fault_allowed =
 		type == KVM_X86_DEFAULT_VM || type == KVM_X86_SW_PROTECTED_VM;
-- 
2.50.0.727.gbf7dc18ff4-goog