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Re: [Qemu-devel] [RFC] virtio: convert to use DMA api
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [RFC] virtio: convert to use DMA api |
Date: |
Mon, 23 Nov 2015 11:52:50 +0200 |
On Mon, Nov 23, 2015 at 10:36:45AM +0100, Cornelia Huck wrote:
> On Mon, 23 Nov 2015 15:41:11 +0800
> Jason Wang <address@hidden> wrote:
>
> > Currently, all virtio devices bypass IOMMU completely. This is because
> > address_space_memory is assumed and used during DMA emulation. This
> > patch converts the virtio core API to use DMA API. This idea is
> >
> > - introducing a new transport specific helper to query the dma address
> > space. (only pci version is implemented).
> > - query and use this address space during virtio device guest memory
> > accessing
> >
> > With this virtiodevices will not bypass IOMMU anymore. Little tested with
> > intel_iommu=on with virtio guest DMA series posted in
> > https://lkml.org/lkml/2015/10/28/64.
> >
> > TODO:
> > - Feature bit for this
>
> I'm still not convinced about that feature bit stuff. It just feels
> wrong to use a mechanism that conveys negotiable device features to
> configure what is basically a platform/hypervisor feature. I'd rather
> see this out of the virtio layer and into the pci layer.
I agree it's not something that needs to be negotiated.
But given that we are changing device and driver behaviour in a drastic
way, it seems prudent to have a way for guest and host to discover that,
even if it's just in case.
Whether it's a feature bit or something else pci-specific,
depends on whether this makes sense for any other transport.
> > - Implement this for all transports
>
> Is it OK to just keep a fallback to today's implementation for
> transports for which the iommu concept doesn't make sense and that will
> always have an identity mapping?
Is there a reason why iommu does not make any sense for ccw or mmio?
> >
> > Signed-off-by: Jason Wang <address@hidden>
> > ---
> > hw/block/virtio-blk.c | 2 +-
> > hw/char/virtio-serial-bus.c | 2 +-
> > hw/scsi/virtio-scsi.c | 2 +-
> > hw/virtio/virtio-pci.c | 9 +++++++++
> > hw/virtio/virtio.c | 36 +++++++++++++++++++--------------
> > include/hw/virtio/virtio-access.h | 42
> > +++++++++++++++++++++++++++++----------
> > include/hw/virtio/virtio-bus.h | 1 +
> > include/hw/virtio/virtio.h | 2 +-
> > 8 files changed, 67 insertions(+), 29 deletions(-)
>
> FWIW, this doesn't seem to break for the s390-ccw-virtio machine (only
> snifftested).