Re: [PATCH 0/2] virtio: decouple protected guest RAM form VIRTIO_F_IOMMU_PLATFORM

[Halil Pasic]

On Thu, 20 Feb 2020 16:29:50 -0500
"Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:

> On Thu, Feb 20, 2020 at 05:06:04PM +0100, Halil Pasic wrote:
> > * This usage is not congruent with  standardised semantics of
> > VIRTIO_F_IOMMU_PLATFORM. Guest memory protected is an orthogonal reason
> > for using DMA API in virtio (orthogonal with respect to what is
> > expressed by VIRTIO_F_IOMMU_PLATFORM). 
> 
> Quoting the spec:
> 
>   \item[VIRTIO_F_ACCESS_PLATFORM(33)] This feature indicates that
>   the device can be used on a platform where device access to data
>   in memory is limited and/or translated. E.g. this is the case if the device can be located
>   behind an IOMMU that translates bus addresses from the device into physical
>   addresses in memory, if the device can be limited to only access
>   certain memory addresses or if special commands such as
>   a cache flush can be needed to synchronise data in memory with
>   the device. Whether accesses are actually limited or translated
>   is described by platform-specific means.
>   If this feature bit is set to 0, then the device
>   has same access to memory addresses supplied to it as the
>   driver has.
>   In particular, 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, and can access any address supplied to it by
>   the driver. When clear, this overrides any platform-specific description of
>   whether device access is limited or translated in any way, e.g.
>   whether an IOMMU may be present.
> 
> since device can't access encrypted memory,
> this seems to match your case reasonably well.
> 

As David already explained, the device does not have to access encrypted
memory. In fact, we don't have memory encryption but memory protection on
s390. All the device *should* ever see is non-protected memory (one that
was previously shared by the guest).

Our protected guests start as non-protected ones, and may or may not
turn protected during their life-span. From the device perspective,
really, nothing changes. I believe David explained this much better than
I did.

Regards,
Halil