xen/mapcache: store dma information in revmapcache entries for debugging

The Xen mapcache is able to create long term mappings, they are called
"locked" mappings. The third parameter of the xen_map_cache call
specifies if a mapping is a "locked" mapping.

>From the QEMU point of view there are two kinds of long term mappings:

[a] device memory mappings, such as option roms and video memory
[b] dma mappings, created by dma_memory_map & friends

After certain operations, ballooning a VM in particular, Xen asks QEMU
kindly to destroy all mappings. However, certainly [a] mappings are
present and cannot be removed. That's not a problem as they are not
affected by balloonning. The *real* problem is that if there are any
mappings of type [b], any outstanding dma operations could fail. This is
a known shortcoming. In other words, when Xen asks QEMU to destroy all
mappings, it is an error if any [b] mappings exist.

However today we have no way of distinguishing [a] from [b]. Because of
that, we cannot even print a decent warning.

This patch introduces a new "dma" bool field to MapCacheRev entires, to
remember if a given mapping is for dma or is a long term device memory
mapping. When xen_invalidate_map_cache is called, we print a warning if
any [b] mappings exist. We ignore [a] mappings.

Mappings created by qemu_map_ram_ptr are assumed to be [a], while
mappings created by address_space_map->qemu_ram_ptr_length are assumed
to be [b].

The goal of the patch is to make debugging and system understanding
easier.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>

1 files changed, 4 insertions, 4 deletions

diff --git a/exec.c b/exec.c
index eac6085760..85769e1464 100644
--- a/exec.c
+++ b/exec.c
@@ -2084,10 +2084,10 @@ void *qemu_map_ram_ptr(RAMBlock *ram_block, ram_addr_t addr)
          * In that case just map until the end of the page.
          */
         if (block->offset == 0) {
-            return xen_map_cache(addr, 0, 0);
+            return xen_map_cache(addr, 0, 0, false);
         }
 
-        block->host = xen_map_cache(block->offset, block->max_length, 1);
+        block->host = xen_map_cache(block->offset, block->max_length, 1, false);
     }
     return ramblock_ptr(block, addr);
 }
@@ -2117,10 +2117,10 @@ static void *qemu_ram_ptr_length(RAMBlock *ram_block, ram_addr_t addr,
          * In that case just map the requested area.
          */
         if (block->offset == 0) {
-            return xen_map_cache(addr, *size, 1);
+            return xen_map_cache(addr, *size, 1, true);
         }
 
-        block->host = xen_map_cache(block->offset, block->max_length, 1);
+        block->host = xen_map_cache(block->offset, block->max_length, 1, true);
     }
 
     return ramblock_ptr(block, addr);