[RFC v2 PATCH 05/13] KVM: Implement fd-based memory using new memfd interfaces

Chao Peng
2021-11-19T13:49:21Z

This patch pairs a fd-based memslot to a memory backing store. Two sides
handshake to exchange callbacks that will be called later.

KVM->memfd:
- get_pfn: get or allocate(when alloc is true) page at specified
offset in the fd, the page will be locked
- put_pfn: put and unlock the pfn

memfd->KVM:
- invalidate_page_range: called when userspace punch hole on the fd,
KVM should unmap related pages in the second MMU
- fallocate: called when userspace fallocate space on the fd, KVM
can map related pages in the second MMU

Currently tmpfs behind memfd interface is supported.

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
---
arch/x86/kvm/Makefile | 3 +-
include/linux/kvm_host.h | 6 +++
virt/kvm/memfd.c | 101 +++++++++++++++++++++++++++++++++++++++
3 files changed, 109 insertions(+), 1 deletion(-)
create mode 100644 virt/kvm/memfd.c

diff --git a/arch/x86/kvm/Makefile b/arch/x86/kvm/Makefile
index f919df73e5e3..5d7f289b1ca0 100644
--- a/arch/x86/kvm/Makefile
+++ b/arch/x86/kvm/Makefile
@@ -11,7 +11,8 @@ KVM := ../../../virt/kvm

kvm-y += $(KVM)/kvm_main.o $(KVM)/coalesced_mmio.o \
$(KVM)/eventfd.o $(KVM)/irqchip.o $(KVM)/vfio.o \
- $(KVM)/dirty_ring.o $(KVM)/binary_stats.o
+ $(KVM)/dirty_ring.o $(KVM)/binary_stats.o \
+ $(KVM)/memfd.o
kvm-$(CONFIG_KVM_ASYNC_PF) += $(KVM)/async_pf.o

kvm-y += x86.o emulate.o i8259.o irq.o lapic.o \
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 1d4ac0c9b63b..e8646103356b 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -769,6 +769,12 @@ static inline void kvm_irqfd_exit(void)
{
}
#endif
+
+int kvm_memfd_register(struct kvm *kvm,
+ const struct kvm_userspace_memory_region_ext *mem,
+ struct kvm_memory_slot *slot);
+void kvm_memfd_unregister(struct kvm *kvm, struct kvm_memory_slot *slot);
+
int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
struct module *module);
void kvm_exit(void);
diff --git a/virt/kvm/memfd.c b/virt/kvm/memfd.c
new file mode 100644
index 000000000000..bd930dcb455f
--- /dev/null
+++ b/virt/kvm/memfd.c
@@ -0,0 +1,101 @@
+
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * memfd.c: routines for fd based guest memory backing store
+ * Copyright (c) 2021, Intel Corporation.
+ *
+ * Author:
+ * Chao Peng <chao.p.peng@linux.intel.com>
+ */
+
+#include <linux/kvm_host.h>
+#include <linux/memfd.h>
+const static struct guest_mem_ops *memfd_ops;
+
+static void memfd_invalidate_page_range(struct inode *inode, void *owner,
+ pgoff_t start, pgoff_t end)
+{
+ //!!!We can get here after the owner no longer exists
+}
+
+static void memfd_fallocate(struct inode *inode, void *owner,
+ pgoff_t start, pgoff_t end)
+{
+ //!!!We can get here after the owner no longer exists
+}
+
+static const struct guest_ops memfd_notifier = {
+ .invalidate_page_range = memfd_invalidate_page_range,
+ .fallocate = memfd_fallocate,
+};
+
+static kvm_pfn_t kvm_memfd_get_pfn(struct kvm_memory_slot *slot,
+ struct file *file, gfn_t gfn,
+ bool alloc, int *order)
+{
+ pgoff_t index = gfn - slot->base_gfn +
+ (slot->userspace_addr >> PAGE_SHIFT);
+
+ return memfd_ops->get_lock_pfn(file->f_inode, index, alloc, order);
+}
+
+static void kvm_memfd_put_pfn(kvm_pfn_t pfn)
+{
+ memfd_ops->put_unlock_pfn(pfn);
+}
+
+static struct kvm_memfd_ops kvm_memfd_ops = {
+ .get_pfn = kvm_memfd_get_pfn,
+ .put_pfn = kvm_memfd_put_pfn,
+};
+
+int kvm_memfd_register(struct kvm *kvm,
+ const struct kvm_userspace_memory_region_ext *mem,
+ struct kvm_memory_slot *slot)
+{
+ int ret;
+ struct fd fd = fdget(mem->fd);
+
+ if (!fd.file)
+ return -EINVAL;
+
+ ret = memfd_register_guest(fd.file->f_inode, kvm,
+ &memfd_notifier, &memfd_ops);
+ if (ret)
+ return ret;
+ slot->file = fd.file;
+
+ if (mem->private_fd >= 0) {
+ fd = fdget(mem->private_fd);
+ if (!fd.file) {
+ ret = -EINVAL;
+ goto err;
+ }
+
+ ret = memfd_register_guest(fd.file->f_inode, kvm,
+ &memfd_notifier, &memfd_ops);
+ if (ret)
+ goto err;
+ slot->priv_file = fd.file;
+ }
+
+ slot->memfd_ops = &kvm_memfd_ops;
+ return 0;
+err:
+ kvm_memfd_unregister(kvm, slot);
+ return ret;
+}
+
+void kvm_memfd_unregister(struct kvm *kvm, struct kvm_memory_slot *slot)
+{
+ if (slot->file) {
+ fput(slot->file);
+ slot->file = NULL;
+ }
+
+ if (slot->priv_file) {
+ fput(slot->priv_file);
+ slot->priv_file = NULL;
+ }
+ slot->memfd_ops = NULL;
+}
--
2.17.1