Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted

[Thiago Jung Bauermann]

David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> writes:

> On Sat, Mar 23, 2019 at 05:01:35PM -0400, Michael S. Tsirkin wrote:
>> On Thu, Mar 21, 2019 at 09:05:04PM -0300, Thiago Jung Bauermann wrote:
>> > Michael S. Tsirkin <mst@xxxxxxxxxx> writes:
> [snip]
>> > >> > Is there any justification to doing that beyond someone putting
>> > >> > out slow code in the past?
>> > >>
>> > >> The definition of the ACCESS_PLATFORM flag is generic and captures the
>> > >> notion of memory access restrictions for the device. Unfortunately, on
>> > >> powerpc pSeries guests it also implies that the IOMMU is turned on
>> > >
>> > > IIUC that's really because on pSeries IOMMU is *always* turned on.
>> > > Platform has no way to say what you want it to say
>> > > which is bypass the iommu for the specific device.
>> >
>> > Yes, that's correct. pSeries guests running on KVM are in a gray area
>> > where theoretically they use an IOMMU but in practice KVM ignores it.
>> > It's unfortunate but it's the reality on the ground today. :-/
>
> Um.. I'm not sure what you mean by this.  As far as I'm concerned
> there is always a guest-visible (paravirtualized) IOMMU, and that will
> be backed onto the host IOMMU when necessary.

There is, but vhost will ignore it and directly map the guest memory
when ACCESS_PLATFORM (the flag previously known as IOMMU_PLATFORM) isn't
set. From QEMU's hw/virtio/vhost.c:

static int vhost_dev_has_iommu(struct vhost_dev *dev)
{
    VirtIODevice *vdev = dev->vdev;

    return virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
}

static void *vhost_memory_map(struct vhost_dev *dev, hwaddr addr,
                              hwaddr *plen, int is_write)
{
    if (!vhost_dev_has_iommu(dev)) {
        return cpu_physical_memory_map(addr, plen, is_write);
    } else {
        return (void *)(uintptr_t)addr;
    }
}

--
Thiago Jung Bauermann
IBM Linux Technology Center