[RFC PATCH 2/3] RFC: KVM: add KVM_MEM_DMA

[Chia-I Wu]

When the flag is set, it means the the userspace wants to do DMA
with the memory and the guest will use an appropriate memory type to
access the memory.  The kernel should be prepared to honor the
guest's memory type.

Signed-off-by: Chia-I Wu <olvaffe at gmail.com>
Cc: Gurchetan Singh <gurchetansingh at chromium.org>
Cc: Gerd Hoffmann <kraxel at redhat.com>
---
 Documentation/virt/kvm/api.rst | 17 +++++++++++------
 include/uapi/linux/kvm.h       |  2 ++
 virt/kvm/kvm_main.c            |  6 +++++-
 3 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 97a72a53fa4b..e6a27e6e45c2 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -1237,6 +1237,7 @@ yet and must be cleared on entry.
   /* for kvm_memory_region::flags */
   #define KVM_MEM_LOG_DIRTY_PAGES	(1UL << 0)
   #define KVM_MEM_READONLY	(1UL << 1)
+  #define KVM_MEM_DMA		(1UL << 2)
 
 This ioctl allows the user to create, modify or delete a guest physical
 memory slot.  Bits 0-15 of "slot" specify the slot id and this value
@@ -1264,12 +1265,16 @@ It is recommended that the lower 21 bits of guest_phys_addr and userspace_addr
 be identical.  This allows large pages in the guest to be backed by large
 pages in the host.
 
-The flags field supports two flags: KVM_MEM_LOG_DIRTY_PAGES and
-KVM_MEM_READONLY.  The former can be set to instruct KVM to keep track of
-writes to memory within the slot.  See KVM_GET_DIRTY_LOG ioctl to know how to
-use it.  The latter can be set, if KVM_CAP_READONLY_MEM capability allows it,
-to make a new slot read-only.  In this case, writes to this memory will be
-posted to userspace as KVM_EXIT_MMIO exits.
+The flags field supports these flags: KVM_MEM_LOG_DIRTY_PAGES,
+KVM_MEM_READONLY, and KVM_MEM_DMA.  KVM_MEM_LOG_DIRTY_PAGES can be set to
+instruct KVM to keep track of writes to memory within the slot.  See
+KVM_GET_DIRTY_LOG ioctl to know how to use it.  KVM_MEM_READONLY can be set,
+if KVM_CAP_READONLY_MEM capability allows it, to make a new slot read-only.
+In this case, writes to this memory will be posted to userspace as
+KVM_EXIT_MMIO exits.  KVM_MEM_DMA can be set, if KVM_CAP_DMA_MEM capability
+allows it, to make a new slot support DMA.  It is the userspace's
+responsibility to make sure userspace_addr points at a DMA-able memory and the
+guest's responsibility to map guest_phys_addr with the proper memory type.
 
 When the KVM_CAP_SYNC_MMU capability is available, changes in the backing of
 the memory region are automatically reflected into the guest.  For example, an
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 4b95f9a31a2f..578292e4b072 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -109,6 +109,7 @@ struct kvm_userspace_memory_region {
  */
 #define KVM_MEM_LOG_DIRTY_PAGES	(1UL << 0)
 #define KVM_MEM_READONLY	(1UL << 1)
+#define KVM_MEM_DMA		(1UL << 2)
 
 /* for KVM_IRQ_LINE */
 struct kvm_irq_level {
@@ -1010,6 +1011,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_ARM_NISV_TO_USER 177
 #define KVM_CAP_ARM_INJECT_EXT_DABT 178
 #define KVM_CAP_S390_VCPU_RESETS 179
+#define KVM_CAP_DMA_MEM 180
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 70f03ce0e5c1..a4b6c782a168 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -940,6 +940,9 @@ static int check_memory_region_flags(const struct kvm_userspace_memory_region *m
 #ifdef __KVM_HAVE_READONLY_MEM
 	valid_flags |= KVM_MEM_READONLY;
 #endif
+#ifdef __KVM_HAVE_DMA_MEM
+	valid_flags |= KVM_MEM_DMA;
+#endif
 
 	if (mem->flags & ~valid_flags)
 		return -EINVAL;
@@ -1047,7 +1050,8 @@ int __kvm_set_memory_region(struct kvm *kvm,
 		else { /* Modify an existing slot. */
 			if ((mem->userspace_addr != old.userspace_addr) ||
 			    (npages != old.npages) ||
-			    ((new.flags ^ old.flags) & KVM_MEM_READONLY))
+			    ((new.flags ^ old.flags) &
+			     (KVM_MEM_READONLY | KVM_MEM_DMA)))
 				goto out;
 
 			if (base_gfn != old.base_gfn)
-- 
2.25.0.265.gbab2e86ba0-goog