Re: [PATCH 5/5] virtio: Add bounce DMA ops

[Lu Baolu]

On 2020/4/29 4:41, Michael S. Tsirkin wrote:
> On Tue, Apr 28, 2020 at 11:19:52PM +0530, Srivatsa Vaddagiri wrote:
> > * Michael S. Tsirkin<mst@xxxxxxxxxx>  [2020-04-28 12:17:57]:
> >
> > > Okay, but how is all this virtio specific?  For example, why not allow
> > > separate swiotlbs for any type of device?
> > > For example, this might make sense if a given device is from a
> > > different, less trusted vendor.
> > Is swiotlb commonly used for multiple devices that may be on different trust
> > boundaries (and not behind a hardware iommu)?
> Even a hardware iommu does not imply a 100% security from malicious
> hardware. First lots of people use iommu=pt for performance reasons.
> Second even without pt, unmaps are often batched, and sub-page buffers
> might be used for DMA, so we are not 100% protected at all times.

For untrusted devices, IOMMU is forced on even iommu=pt is used; and
iotlb flush is in strict mode (no batched flushes); ATS is also not
allowed. Swiotlb is used to protect sub-page buffers since IOMMU can
only apply page granularity protection. Swiotlb is now used for devices
from different trust zone.

Best regards,
baolu