This patch enables support for shared memory in guest_memfd, including
mapping that memory from host userspace.
This functionality is gated by the KVM_GMEM_SHARED_MEM Kconfig option,
and enabled for a given instance by the GUEST_MEMFD_FLAG_SUPPORT_SHARED
flag at creation time.
Co-developed-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
---
include/linux/kvm_host.h | 13 +++++++
include/uapi/linux/kvm.h | 1 +
virt/kvm/Kconfig | 4 +++
virt/kvm/guest_memfd.c | 76 ++++++++++++++++++++++++++++++++++++++++
4 files changed, 94 insertions(+)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 80371475818f..640ce714cfb2 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)
+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..e90884f74404 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -128,3 +128,7 @@ 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
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 6db515833f61..7a158789d1df 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -312,7 +312,79 @@ 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;
+}
+
+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;
+
+ if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
+ return VM_FAULT_SIGBUS;
+
+ 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;
+}
+
static struct file_operations kvm_gmem_fops = {
+ .mmap = kvm_gmem_mmap,
.open = generic_file_open,
.release = kvm_gmem_release,
.fallocate = kvm_gmem_fallocate,
@@ -428,6 +500,7 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
}
file->f_flags |= O_LARGEFILE;
+ allow_write_access(file);
inode = file->f_inode;
WARN_ON(file->f_mapping != inode->i_mapping);
@@ -463,6 +536,9 @@ 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;
--
2.49.0.1266.g31b7d2e469-goog
