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

Thiago Jung Bauermann
2019-06-28T01:59:12Z

Michael S. Tsirkin <mst@redhat.com> writes:

> On Mon, Jun 03, 2019 at 10:13:59PM -0300, Thiago Jung Bauermann wrote:
>>
>>
>> Michael S. Tsirkin <mst@redhat.com> writes:
>>
>> > On Wed, Apr 17, 2019 at 06:42:00PM -0300, Thiago Jung Bauermann wrote:
>> >> 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.

Great. Thanks for clarifying.

I don't think this flag would work for guest userspace drivers. Should I
add a note about that in the flag definition?

>> > 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?

Sorry, I was mistaken. We do support VFIO in guests but not for virtio
devices, only for regular PCI devices. In which case they will use
address translation.

>> 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.

Ok, so the guest kernel has to forbid VFIO access on devices where this
flag is advertised.

>> 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.

The fact that VIRTIO_F_ACCESS_PLATFORM_NO_TRANSLATION is set
communicates that knowledge to virtio. There is a shared understanding
between the guest and the host about what this flag being set means.

--
Thiago Jung Bauermann
IBM Linux Technology Center