replace max_msg_size with max_data_xfer_size (#541)

The previously specified max_msg_size had one major issue: it implied a (way too
small) limit on the size of dirty bitmaps that could be requested by a client,
and as a result a hard limit on memory region size. It seemed awkward to attempt
to split up an unmap request instead.

Instead, let most requests and replies be limited by their "natural" limits; for
example, the number of booleans in VFIO_USER_SET_IRQS is limited by MSI-X count.

For the requests that solicit or provide data - that is, VFIO_USER_DMA_READ/WRITE
and VFIO_USER_REGION_READ/WRITE - we negotiate a new max_data_xfer_size value.
These are much easier to split up into separate requests at the client side
so should not present an implementation problem. For our server, chunking is
implemented in vfu_dma_read/vfu_dma_write().

Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Swapnil Ingle <swapnil.ingle@nutanix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@nutanix.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/lib/private.h b/lib/private.h
index c7c0627..f474e1f 100644
--- a/lib/private.h
+++ b/lib/private.h
@@ -37,6 +37,7 @@
 
 #include "common.h"
 #include "pci_caps.h"
+#include "vfio-user.h"
 
 /*
  * The main reason we limit the size of an individual DMA region from the client
@@ -46,7 +47,14 @@
 #define MAX_DMA_SIZE (8 * ONE_TB)
 #define MAX_DMA_REGIONS 16
 
-#define SERVER_MAX_MSG_SIZE 65536
+#define SERVER_MAX_DATA_XFER_SIZE (VFIO_USER_DEFAULT_MAX_DATA_XFER_SIZE)
+
+/*
+ * Enough to receive a VFIO_USER_REGION_WRITE of SERVER_MAX_DATA_XFER_SIZE.
+ */
+#define SERVER_MAX_MSG_SIZE (SERVER_MAX_DATA_XFER_SIZE + \
+                             sizeof(struct vfio_user_header) + \
+                             sizeof(struct vfio_user_region_access))
 
 /*
  * Structure used to hold an in-flight request+reply.
@@ -155,6 +163,7 @@ struct vfu_ctx {
     vfu_dma_unregister_cb_t *dma_unregister;
 
     int                     client_max_fds;
+    size_t                  client_max_data_xfer_size;
 
     struct migration        *migration;