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, 3 insertions, 2 deletions

diff --git a/include/sysemu/xen-mapcache.h b/include/sysemu/xen-mapcache.h
index b8c93b9bce..01daaad00c 100644
--- a/include/sysemu/xen-mapcache.h
+++ b/include/sysemu/xen-mapcache.h
@@ -17,7 +17,7 @@ typedef hwaddr (*phys_offset_to_gaddr_t)(hwaddr start_addr,
 void xen_map_cache_init(phys_offset_to_gaddr_t f,
                         void *opaque);
 uint8_t *xen_map_cache(hwaddr phys_addr, hwaddr size,
-                       uint8_t lock);
+                       uint8_t lock, bool dma);
 ram_addr_t xen_ram_addr_from_mapcache(void *ptr);
 void xen_invalidate_map_cache_entry(uint8_t *buffer);
 void xen_invalidate_map_cache(void);
@@ -31,7 +31,8 @@ static inline void xen_map_cache_init(phys_offset_to_gaddr_t f,
 
 static inline uint8_t *xen_map_cache(hwaddr phys_addr,
                                      hwaddr size,
-                                     uint8_t lock)
+                                     uint8_t lock,
+                                     bool dma)
 {
     abort();
 }