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

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 31 May 2019 15:28:07 +0000
Thanos Makatos <thanos.makatos@nutanix.com> wrote:

> > > 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;
> +

Nope, this would prevent IOMMU mapping of assigned device MMIO regions
which would prevent peer-to-peer DMA between assigned devices.  Thanks,

Alex