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 (shared memory). To that end, this patch adds
the ability to check whether the VM type has that support, and
only allows mapping its memory if that's the case.
Additionally, this behavior is gated with a new configuration
option, CONFIG_KVM_GMEM_SHARED_MEM.
Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
---
This patch series will allow shared memory support for software
VMs in x86. It will also introduce a similar VM type for arm64
and allow shared memory support for that. In the future, pKVM
will also support shared memory.
---
include/linux/kvm_host.h | 11 +++++
virt/kvm/Kconfig | 4 ++
virt/kvm/guest_memfd.c | 93 ++++++++++++++++++++++++++++++++++++++++
3 files changed, 108 insertions(+)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 8b5f28f6efff..438aa3df3175 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -728,6 +728,17 @@ static inline bool kvm_arch_has_private_mem(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_PRIVATE_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)
{
diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index 54e959e7d68f..4e759e8020c5 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -124,3 +124,7 @@ config HAVE_KVM_ARCH_GMEM_PREPARE
config HAVE_KVM_ARCH_GMEM_INVALIDATE
bool
depends on KVM_PRIVATE_MEM
+
+config KVM_GMEM_SHARED_MEM
+ select KVM_PRIVATE_MEM
+ bool
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index c6f6792bec2a..85467a3ef8ea 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -317,9 +317,102 @@ void kvm_gmem_handle_folio_put(struct folio *folio)
{
WARN_ONCE(1, "A placeholder that shouldn't trigger. Work in progress.");
}
+
+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(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)) {
+ ret = VM_FAULT_SIGBUS;
+ goto out_filemap;
+ }
+
+ if (folio_test_hwpoison(folio)) {
+ ret = VM_FAULT_HWPOISON;
+ goto out_folio;
+ }
+
+ /* Must be called with folio lock held, i.e., after kvm_gmem_get_folio() */
+ if (!kvm_gmem_offset_is_shared(vmf->vma->vm_file, vmf->pgoff)) {
+ ret = VM_FAULT_SIGBUS;
+ goto out_folio;
+ }
+
+ /*
+ * Only private folios are marked as "guestmem" so far, and we never
+ * expect private folios at this point.
+ */
+ if (WARN_ON_ONCE(folio_test_guestmem(folio))) {
+ ret = VM_FAULT_SIGBUS;
+ goto out_folio;
+ }
+
+ /* No support for huge pages. */
+ 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,
+};
+
+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;
+ }
+
+ file_accessed(file);
+ 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,
--
2.48.1.502.g6dc24dfdaf-goog
