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Re: [Qemu-devel] QEMU tries to register to VFIO memory that is not RAM
From: |
Alex Williamson |
Subject: |
Re: [Qemu-devel] QEMU tries to register to VFIO memory that is not RAM |
Date: |
Fri, 31 May 2019 09:40:02 -0600 |
On Fri, 31 May 2019 15:28:07 +0000
Thanos Makatos <address@hidden> 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