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

On Mon, Jun 03, 2019 at 10:13:59PM -0300, Thiago Jung Bauermann wrote:

Michael S. Tsirkin [off-list ref] writes:

quoted

On Wed, Apr 17, 2019 at 06:42:00PM -0300, Thiago Jung Bauermann wrote:

quoted

I rephrased it in terms of address translation. What do you think of
this version? The flag name is slightly different too:


VIRTIO_F_ACCESS_PLATFORM_NO_TRANSLATION This feature has the same
    meaning as VIRTIO_F_ACCESS_PLATFORM both when set and when not set,
    with the exception that address translation is guaranteed to be
    unnecessary when accessing memory addresses supplied to the device
    by the driver. Which is to say, the device will always use physical
    addresses matching addresses used by the driver (typically meaning
    physical addresses used by the CPU) and not translated further. This
    flag should be set by the guest if offered, but to allow for
    backward-compatibility device implementations allow for it to be
    left unset by the guest. It is an error to set both this flag and
    VIRTIO_F_ACCESS_PLATFORM.

OK so VIRTIO_F_ACCESS_PLATFORM is designed to allow unpriveledged
drivers. This is why devices fail when it's not negotiated.

Just to clarify, what do you mean by unprivileged drivers? Is it drivers
implemented in guest userspace such as with VFIO? Or unprivileged in
some other sense such as needing to use bounce buffers for some reason?

I had drivers in guest userspace in mind.

quoted

This confuses me.
If driver is unpriveledged then what happens with this flag?
It can supply any address it wants. Will that corrupt kernel
memory?

Not needing address translation doesn't necessarily mean that there's no
IOMMU. On powerpc we don't use VIRTIO_F_ACCESS_PLATFORM but there's
always an IOMMU present. And we also support VFIO drivers. The VFIO API
for pseries (sPAPR section in Documentation/vfio.txt) has extra ioctls
to program the IOMMU.

For our use case, we don't need address translation because we set up an
identity mapping in the IOMMU so that the device can use guest physical
addresses.

And can it access any guest physical address?

If the guest kernel is concerned that an unprivileged driver could
jeopardize its integrity it should not negotiate this feature flag.

Unfortunately flag negotiation is done through config space
and so can be overwritten by the driver.

Perhaps there should be a note about this in the flag definition? This
concern is platform-dependant though. I don't believe it's an issue in
pseries.

Again ACCESS_PLATFORM has a pretty open definition. It does actually
say it's all up to the platform.

Specifically how will VIRTIO_F_ACCESS_PLATFORM_NO_TRANSLATION be
implemented portably? virtio has no portable way to know
whether DMA API bypasses translation.

--
Thiago Jung Bauermann
IBM Linux Technology Center