[PATCH RFC 03/12] vfio/migration: Throttle vfio_save_block() on data size to read

[Peter Xu <peterx@redhat.com>]

During precopy phase, VFIO maintains two counters for init/dirty data
tracking for query estimations.

VFIO fetches data during precopy by reading from the VFIO fd, after
fetching it'll deduct the read size.

Here since the fd's size can dynamically change, I think it means VFIO may
read more than what it "thought" were there for fetching.

I highly suspect it's also relevant to a weird case in the function of
vfio_update_estimated_pending_data(), where when VFIO reads 0 from the FD
it will _reset_ the two counters, instead of asserting both of them being
zeros, which looks pretty hackish.

Just guarantee it from userspace level that VFIO won't read more than what
it expects for now.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 hw/vfio/migration.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
index 83327b6573..851ea783f3 100644
--- a/hw/vfio/migration.c
+++ b/hw/vfio/migration.c
@@ -357,12 +357,18 @@ static int vfio_query_precopy_size(VFIOMigration *migration)
 }
 
 /* Returns the size of saved data on success and -errno on error */
-static ssize_t vfio_save_block(QEMUFile *f, VFIOMigration *migration)
+static ssize_t vfio_save_block(QEMUFile *f, VFIOMigration *migration,
+                               bool precopy)
 {
-    ssize_t data_size;
+    ssize_t data_size = migration->data_buffer_size;
+
+    if (precopy) {
+        /* Limit the buffer size to make sure cached stats don't overflow */
+        data_size = MIN(data_size, migration->precopy_init_size +
+                        migration->precopy_dirty_size);
+    }
 
-    data_size = read(migration->data_fd, migration->data_buffer,
-                     migration->data_buffer_size);
+    data_size = read(migration->data_fd, migration->data_buffer, data_size);
     if (data_size < 0) {
         /*
          * Pre-copy emptied all the device state for now. For more information,
@@ -623,7 +629,7 @@ static int vfio_save_iterate(QEMUFile *f, void *opaque)
         migration->event_save_iterate_started = true;
     }
 
-    data_size = vfio_save_block(f, migration);
+    data_size = vfio_save_block(f, migration, true);
     if (data_size < 0) {
         return data_size;
     }
@@ -667,7 +673,7 @@ static int vfio_save_complete_precopy(QEMUFile *f, void *opaque)
     }
 
     do {
-        data_size = vfio_save_block(f, vbasedev->migration);
+        data_size = vfio_save_block(f, vbasedev->migration, false);
         if (data_size < 0) {
             return data_size;
         }
-- 
2.50.1