Drivers: hv: vmbus: Add vmbus_alloc_buffer()/vmbus_free_buffer() for CoCo VMs

On CoCo VMs without confidential VMBus, the netvsc send and receive buffers
must be made host-visible by decrypting them. These buffers are vmalloc'ed,
but set_memory_decrypted()/encrypted() do not work on vmalloc'ed memory.
This use case is (so far) unique to netvsc, so solve it locally rather than
changing the set_memory() or allocation APIs.

Add vmbus_alloc_buffer()/vmbus_free_buffer() to the VMBus core. When the
guest's isolation model requires it, allocate the buffer as a list of
physically-contiguous chunks via alloc_pages_node(), starting at
MAX_PAGE_ORDER and falling back to smaller orders so the allocation still
succeeds under memory fragmentation. Each chunk is decrypted in place via
set_memory_decrypted() on its direct-map address, and the chunks are then
stitched into a single virtually-contiguous range with vmap(). Buffers that
do not need decryption keep using vzalloc().

To free the buffer, vmbus_free_buffer() calls vunmap() on the range then
re-encrypts and frees each chunk individually; any chunk that fails
re-encryption is leaked to prevent accidentally freeing decrypted memory.

This approach minimizes scattering of decrypted 4 KiB pages through the
kernel direct map and the resulting shattering of large page mappings.

Signed-off-by: Kameron Carr <kameroncarr@linux.microsoft.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 08b927fab610..a97be78653e8 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -1214,6 +1214,13 @@ extern int vmbus_establish_gpadl_caller_decrypted(struct vmbus_channel *channel,
 extern int vmbus_teardown_gpadl(struct vmbus_channel *channel,
 				     struct vmbus_gpadl *gpadl);
 
+extern void *vmbus_alloc_buffer(struct vmbus_channel *channel,
+				u32 size,
+				struct page ***chunks_out,
+				u32 *chunk_cnt_out);
+
+extern void vmbus_free_buffer(void *addr, struct page **chunks, u32 chunk_cnt);
+
 void vmbus_reset_channel_cb(struct vmbus_channel *channel);
 
 extern int vmbus_recvpacket(struct vmbus_channel *channel,