[PATCH v10 08/16] KVM: guest_memfd: Allow host to map guest_memfd pages

[Fuad Tabba <tabba@google.com>]

This patch enables support for shared memory in guest_memfd, including
mapping that memory at the host userspace. This support is gated by the
configuration option KVM_GMEM_SHARED_MEM, and toggled by the guest_memfd
flag GUEST_MEMFD_FLAG_SUPPORT_SHARED, which can be set when creating a
guest_memfd instance.

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 | 10 ++++
 arch/x86/kvm/x86.c              |  3 +-
 include/linux/kvm_host.h        | 13 ++++++
 include/uapi/linux/kvm.h        |  1 +
 virt/kvm/Kconfig                |  5 ++
 virt/kvm/guest_memfd.c          | 81 +++++++++++++++++++++++++++++++++
 6 files changed, 112 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 709cc2a7ba66..ce9ad4cd93c5 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2255,8 +2255,18 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
 
 #ifdef CONFIG_KVM_GMEM
 #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_shared_mem(kvm)			\
+	(IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM) &&			\
+	 ((kvm)->arch.vm_type == KVM_X86_SW_PROTECTED_VM ||		\
+	  (kvm)->arch.vm_type == KVM_X86_DEFAULT_VM))
 #else
 #define kvm_arch_supports_gmem(kvm) false
+#define kvm_arch_supports_gmem_shared_mem(kvm) false
 #endif
 
 #define kvm_arch_has_readonly_mem(kvm) (!(kvm)->arch.has_protected_state)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 035ced06b2dd..2a02f2457c42 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12718,7 +12718,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 		return -EINVAL;
 
 	kvm->arch.vm_type = type;
-	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;
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 80371475818f..ba83547e62b0 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -729,6 +729,19 @@ static inline bool kvm_arch_supports_gmem(struct kvm *kvm)
 }
 #endif
 
+/*
+ * Returns true if this VM supports shared mem in guest_memfd.
+ *
+ * Arch code must define kvm_arch_supports_gmem_shared_mem if support for
+ * guest_memfd is enabled.
+ */
+#if !defined(kvm_arch_supports_gmem_shared_mem) && !IS_ENABLED(CONFIG_KVM_GMEM)
+static inline bool kvm_arch_supports_gmem_shared_mem(struct kvm *kvm)
+{
+	return false;
+}
+#endif
+
 #ifndef kvm_arch_has_readonly_mem
 static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm)
 {
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index b6ae8ad8934b..c2714c9d1a0e 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1566,6 +1566,7 @@ struct kvm_memory_attributes {
 #define KVM_MEMORY_ATTRIBUTE_PRIVATE           (1ULL << 3)
 
 #define KVM_CREATE_GUEST_MEMFD	_IOWR(KVMIO,  0xd4, struct kvm_create_guest_memfd)
+#define GUEST_MEMFD_FLAG_SUPPORT_SHARED	(1ULL << 0)
 
 struct kvm_create_guest_memfd {
 	__u64 size;
diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index 559c93ad90be..df225298ab10 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -128,3 +128,8 @@ config HAVE_KVM_ARCH_GMEM_PREPARE
 config HAVE_KVM_ARCH_GMEM_INVALIDATE
        bool
        depends on KVM_GMEM
+
+config KVM_GMEM_SHARED_MEM
+       select KVM_GMEM
+       bool
+       prompt "Enable support for non-private (shared) memory in guest_memfd"
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 6db515833f61..5d34712f64fc 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -312,7 +312,81 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
 	return gfn - slot->base_gfn + slot->gmem.pgoff;
 }
 
+static bool kvm_gmem_supports_shared(struct inode *inode)
+{
+	u64 flags;
+
+	if (!IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM))
+		return false;
+
+	flags = (u64)inode->i_private;
+
+	return flags & GUEST_MEMFD_FLAG_SUPPORT_SHARED;
+}
+
+
+#ifdef CONFIG_KVM_GMEM_SHARED_MEM
+static vm_fault_t kvm_gmem_fault_shared(struct vm_fault *vmf)
+{
+	struct inode *inode = file_inode(vmf->vma->vm_file);
+	struct folio *folio;
+	vm_fault_t ret = VM_FAULT_LOCKED;
+
+	folio = kvm_gmem_get_folio(inode, vmf->pgoff);
+	if (IS_ERR(folio)) {
+		int err = PTR_ERR(folio);
+
+		if (err == -EAGAIN)
+			return VM_FAULT_RETRY;
+
+		return vmf_error(err);
+	}
+
+	if (WARN_ON_ONCE(folio_test_large(folio))) {
+		ret = VM_FAULT_SIGBUS;
+		goto out_folio;
+	}
+
+	if (!folio_test_uptodate(folio)) {
+		clear_highpage(folio_page(folio, 0));
+		kvm_gmem_mark_prepared(folio);
+	}
+
+	vmf->page = folio_file_page(folio, vmf->pgoff);
+
+out_folio:
+	if (ret != VM_FAULT_LOCKED) {
+		folio_unlock(folio);
+		folio_put(folio);
+	}
+
+	return ret;
+}
+
+static const struct vm_operations_struct kvm_gmem_vm_ops = {
+	.fault = kvm_gmem_fault_shared,
+};
+
+static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	if (!kvm_gmem_supports_shared(file_inode(file)))
+		return -ENODEV;
+
+	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) !=
+	    (VM_SHARED | VM_MAYSHARE)) {
+		return -EINVAL;
+	}
+
+	vma->vm_ops = &kvm_gmem_vm_ops;
+
+	return 0;
+}
+#else
+#define kvm_gmem_mmap NULL
+#endif /* CONFIG_KVM_GMEM_SHARED_MEM */
+
 static struct file_operations kvm_gmem_fops = {
+	.mmap		= kvm_gmem_mmap,
 	.open		= generic_file_open,
 	.release	= kvm_gmem_release,
 	.fallocate	= kvm_gmem_fallocate,
@@ -463,6 +537,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
 	u64 flags = args->flags;
 	u64 valid_flags = 0;
 
+	if (kvm_arch_supports_gmem_shared_mem(kvm))
+		valid_flags |= GUEST_MEMFD_FLAG_SUPPORT_SHARED;
+
 	if (flags & ~valid_flags)
 		return -EINVAL;
 
@@ -501,6 +578,10 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 	    offset + size > i_size_read(inode))
 		goto err;
 
+	if (kvm_gmem_supports_shared(inode) &&
+	    !kvm_arch_supports_gmem_shared_mem(kvm))
+		goto err;
+
 	filemap_invalidate_lock(inode->i_mapping);
 
 	start = offset >> PAGE_SHIFT;
-- 
2.49.0.1164.gab81da1b16-goog