Re: [Qemu-devel] QEMU tries to register to VFIO memory that is not RAM

[Thanos Makatos]

> > When configuring device pass-through via VFIO to a VM, I noticed that
> > QEMU tries to register (DMA_MAP) all memory regions of a guest (not
                > > only RAM). That includes firmware regions like "pc.rom". 
Would a
> > physical device ever need access to those?
>
> Probably not, but are those things not in the address space of the
> device on a physical system?

They are. I'm wondering whether it makes sense in a virtualized environment.

>
> > Am I missing something?
>
> Does this cause a problem?

It does in my use case. We're experimenting with devices backed by another
userspace application. We can configure QEMU to allocate shared memory
(MAP_SHARED) for guest RAM (which we can register in the other process) but not
for anything else.

>  It's not always easy to identify regions
> that should not be mapped to a device, clearly we're not going to
> create a whitelist based on the name of the region.  Thanks,

Indeed. Could we decide whether or not to register an address space with
VFIO in a more intelligent manner? E.g. the following simplistic patch solves
our problem:

diff --git a/hw/vfio/common.c b/hw/vfio/common.c
index 4374cc6176..d9d3b1277a 100644
--- a/hw/vfio/common.c
+++ b/hw/vfio/common.c
@@ -430,6 +430,9 @@ static void vfio_listener_region_add(MemoryListener 
*listener,
     VFIOHostDMAWindow *hostwin;
     bool hostwin_found;

+    if (!section->mr->ram_device)
+        return;
+
     if (vfio_listener_skipped_section(section)) {
         trace_vfio_listener_region_add_skip(
                 section->offset_within_address_space,