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

[Thiago Jung Bauermann]

Michael S. Tsirkin <mst@xxxxxxxxxx> writes:

> On Mon, Jul 15, 2019 at 07:03:03PM -0300, Thiago Jung Bauermann wrote:
>> 
>> Michael S. Tsirkin <mst@xxxxxxxxxx> writes:
>> 
>> > On Mon, Jul 15, 2019 at 05:29:06PM -0300, Thiago Jung Bauermann wrote:
>> >>
>> >> Michael S. Tsirkin <mst@xxxxxxxxxx> writes:
>> >>
>> >> > On Sun, Jul 14, 2019 at 02:51:18AM -0300, Thiago Jung Bauermann wrote:
>> >> >>
>> >> >>
>> >> >> Michael S. Tsirkin <mst@xxxxxxxxxx> writes:
>> >> >>
>> >> >> > So this is what I would call this option:
>> >> >> >
>> >> >> > VIRTIO_F_ACCESS_PLATFORM_IDENTITY_ADDRESS
>> >> >> >
>> >> >> > and the explanation should state that all device
>> >> >> > addresses are translated by the platform to identical
>> >> >> > addresses.
>> >> >> >
>> >> >> > In fact this option then becomes more, not less restrictive
>> >> >> > than VIRTIO_F_ACCESS_PLATFORM - it's a promise
>> >> >> > by guest to only create identity mappings,
>> >> >> > and only before driver_ok is set.
>> >> >> > This option then would always be negotiated together with
>> >> >> > VIRTIO_F_ACCESS_PLATFORM.
>> >> >> >
>> >> >> > Host then must verify that
>> >> >> > 1. full 1:1 mappings are created before driver_ok
>> >> >> >     or can we make sure this happens before features_ok?
>> >> >> >     that would be ideal as we could require that features_ok fails
>> >> >> > 2. mappings are not modified between driver_ok and reset
>> >> >> >     i guess attempts to change them will fail -
>> >> >> >     possibly by causing a guest crash
>> >> >> >     or some other kind of platform-specific error
>> >> >>
>> >> >> I think VIRTIO_F_ACCESS_PLATFORM_IDENTITY_ADDRESS is good, but requiring
>> >> >> it to be accompanied by ACCESS_PLATFORM can be a problem. One reason is
>> >> >> SLOF as I mentioned above, another is that we would be requiring all
>> >> >> guests running on the machine (secure guests or not, since we would use
>> >> >> the same configuration for all guests) to support it. But
>> >> >> ACCESS_PLATFORM is relatively recent so it's a bit early for that. For
>> >> >> instance, Ubuntu 16.04 LTS (which is still supported) doesn't know about
>> >> >> it and wouldn't be able to use the device.
>> >> >
>> >> > OK and your target is to enable use with kernel drivers within
>> >> > guests, right?
>> >>
>> >> Right.
>> >>
>> >> > My question is, we are defining a new flag here, I guess old guests
>> >> > then do not set it. How does it help old guests? Or maybe it's
>> >> > not designed to ...
>> >>
>> >> Indeed. The idea is that QEMU can offer the flag, old guests can reject
>> >> it (or even new guests can reject it, if they decide not to convert into
>> >> secure VMs) and the feature negotiation will succeed with the flag
>> >> unset.
>> >
>> > OK. And then what does QEMU do? Assume guest is not encrypted I guess?
>> 
>> There's nothing different that QEMU needs to do, with or without the
>> flag. the perspective of the host, a secure guest and a regular guest
>> work the same way with respect to virtio.
>
> OK. So now let's get back to implementation. What will
> Linux guest driver do? It can't activate DMA API blindly since that
> will assume translation also works, right?

It can on pseries, because we always have a 1:1 window mapping the whole
guest memory.

> Or do we somehow limit it to just a specific platform?

Yes, we want to accept the new flag only on secure pseries guests.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center