support for shadow ioeventfd (#698)

When an ioeventfd is written to, KVM discards the value since it has no
memory to write it to, and simply kicks the eventfd. This a problem for
devices such a NVMe controllers that need the value (e.g. doorbells on
BAR0). This patch allows the vfio-user server to pass a file descriptor
that can be mmap'ed and KVM can write the ioeventfd value to this
_shadow_ memory instead of discarding it. This shadow memory is not
exposed to the guest.

Signed-off-by: Thanos Makatos <thanos.makatos@nutanix.com>
Reviewed-by: John Levon <john.levon@nutanix.com>
Change-Id: Iad849c94076ffa5988e034c8bf7ec312d01f095f

1 files changed, 1 insertions, 0 deletions

diff --git a/lib/private.h b/lib/private.h
index 7ffd6be..b875138 100644
--- a/lib/private.h
+++ b/lib/private.h
@@ -186,6 +186,7 @@ typedef struct ioeventfd {
     int32_t fd;
     uint32_t flags;
     uint64_t datamatch;
+    int32_t shadow_fd;
     LIST_ENTRY(ioeventfd) entry;
 } ioeventfd_t;