hostmem-file: add offset option

Add an option for hostmem-file to start the memory object at an offset
into the target file. This is useful if multiple memory objects reside
inside the same target file, such as a device node.

In particular, it's useful to map guest memory directly into /dev/mem
for experimentation.

To make this work consistently, also fix up all places in QEMU that
expect fd offsets to be 0.

Signed-off-by: Alexander Graf <graf@amazon.com>
Message-Id: <20230403221421.60877-1-graf@amazon.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index e3ec8727da..74a2a28663 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -473,6 +473,7 @@ static MemoryRegion *vhost_user_get_mr_data(uint64_t addr, ram_addr_t *offset,
     assert((uintptr_t)addr == addr);
     mr = memory_region_from_host((void *)(uintptr_t)addr, offset);
     *fd = memory_region_get_fd(mr);
+    *offset += mr->ram_block->fd_offset;
 
     return mr;
 }