[PATCH v8 08/13] KVM: guest_memfd: Allow host to map guest_memfd() pages

[Fuad Tabba <tabba@google.com>]

Add support for mmap() and fault() for guest_memfd backed memory
in the host for VMs that support in-place conversion between
shared and private. To that end, this patch adds the ability to
check whether the VM type supports in-place conversion, and only
allows mapping its memory if that's the case.

This patch introduces the configuration option KVM_GMEM_SHARED_MEM,
which enables support for in-place shared memory.

It also introduces the KVM capability KVM_CAP_GMEM_SHARED_MEM, which
indicates that the host can create VMs that support shared memory.
Supporting shared memory implies that memory can be mapped when shared
with the host.

Signed-off-by: Fuad Tabba <tabba@google.com>
---
 include/linux/kvm_host.h | 15 ++++++-
 include/uapi/linux/kvm.h |  1 +
 virt/kvm/Kconfig         |  5 +++
 virt/kvm/guest_memfd.c   | 92 ++++++++++++++++++++++++++++++++++++++++
 virt/kvm/kvm_main.c      |  4 ++
 5 files changed, 116 insertions(+), 1 deletion(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 9419fb99f7c2..f3af6bff3232 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -729,6 +729,17 @@ static inline bool kvm_arch_supports_gmem(struct kvm *kvm)
 }
 #endif
 
+/*
+ * Arch code must define kvm_arch_gmem_supports_shared_mem if support for
+ * private memory is enabled and it supports in-place shared/private conversion.
+ */
+#if !defined(kvm_arch_gmem_supports_shared_mem) && !IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM)
+static inline bool kvm_arch_gmem_supports_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)
 {
@@ -2516,7 +2527,9 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
 
 static inline bool kvm_mem_from_gmem(struct kvm *kvm, gfn_t gfn)
 {
-	/* For now, only private memory gets consumed from guest_memfd. */
+	if (kvm_arch_gmem_supports_shared_mem(kvm))
+		return true;
+
 	return kvm_mem_is_private(kvm, gfn);
 }
 
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index b6ae8ad8934b..8bc8046c7f3a 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/Kconfig b/virt/kvm/Kconfig
index 559c93ad90be..f4e469a62a60 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 "Enables in-place shared memory for guest_memfd"
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 6db515833f61..8bc8fc991d58 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -312,7 +312,99 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
 	return gfn - slot->base_gfn + slot->gmem.pgoff;
 }
 
+#ifdef CONFIG_KVM_GMEM_SHARED_MEM
+/*
+ * Returns true if the folio is shared with the host and the guest.
+ */
+static bool kvm_gmem_offset_is_shared(struct file *file, pgoff_t index)
+{
+	struct kvm_gmem *gmem = file->private_data;
+
+	/* For now, VMs that support shared memory share all their memory. */
+	return kvm_arch_gmem_supports_shared_mem(gmem->kvm);
+}
+
+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;
+
+	filemap_invalidate_lock_shared(inode->i_mapping);
+
+	folio = kvm_gmem_get_folio(inode, vmf->pgoff);
+	if (IS_ERR(folio)) {
+		int err = PTR_ERR(folio);
+
+		if (err == -EAGAIN)
+			ret = VM_FAULT_RETRY;
+		else
+			ret = vmf_error(err);
+
+		goto out_filemap;
+	}
+
+	if (folio_test_hwpoison(folio)) {
+		ret = VM_FAULT_HWPOISON;
+		goto out_folio;
+	}
+
+	if (!kvm_gmem_offset_is_shared(vmf->vma->vm_file, vmf->pgoff)) {
+		ret = VM_FAULT_SIGBUS;
+		goto out_folio;
+	}
+
+	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);
+	}
+
+out_filemap:
+	filemap_invalidate_unlock_shared(inode->i_mapping);
+
+	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)
+{
+	struct kvm_gmem *gmem = file->private_data;
+
+	if (!kvm_arch_gmem_supports_shared_mem(gmem->kvm))
+		return -ENODEV;
+
+	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) !=
+	    (VM_SHARED | VM_MAYSHARE)) {
+		return -EINVAL;
+	}
+
+	vm_flags_set(vma, VM_DONTDUMP);
+	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,
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 6289ea1685dd..c75d8e188eb7 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4845,6 +4845,10 @@ 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_gmem_supports_shared_mem(kvm);
 #endif
 	default:
 		break;
-- 
2.49.0.901.g37484f566f-goog