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SubjectRe: [PATCH 0/2] virtio: Support encrypted memory on powerpc secure guests
Hmm.. git-send-email forgot to CC  Michael Tsirkin, and Thiago; the
original author, who is on vacation.

Adding them now.
RP

On Fri, Oct 11, 2019 at 06:25:17PM -0700, Ram Pai wrote:
> **We would like the patches to be merged through the virtio tree. Please
> review, and ack merging the DMA mapping change through that tree. Thanks!**
>
> The memory of powerpc secure guests can't be accessed by the hypervisor /
> virtio device except for a few memory regions designated as 'shared'.
>
> At the moment, Linux uses bounce-buffering to communicate with the
> hypervisor, with a bounce buffer marked as shared. This is how the DMA API
> is implemented on this platform.
>
> In particular, the most convenient way to use virtio on this platform is by
> making virtio use the DMA API: in fact, this is exactly what happens if the
> virtio device exposes the flag VIRTIO_F_ACCESS_PLATFORM. However, bugs in the
> hypervisor on the powerpc platform do not allow setting this flag, with some
> hypervisors already in the field that don't set this flag. At the moment they
> are forced to use emulated devices when guest is in secure mode; virtio is
> only useful when guest is not secure.
>
> Normally, both device and driver must support VIRTIO_F_ACCESS_PLATFORM:
> if one of them doesn't, the other mustn't assume it for communication
> to work.
>
> However, a guest-side work-around is possible to enable virtio
> for these hypervisors with guest in secure mode: it so happens that on
> powerpc secure platform the DMA address is actually a physical address -
> that of the bounce buffer. For these platforms we can make the virtio
> driver go through the DMA API even though the device itself ignores
> the DMA API.
>
> These patches implement this work around for virtio: we detect that
> - secure guest mode is enabled - so we know that since we don't share
> most memory and Hypervisor has not enabled VIRTIO_F_ACCESS_PLATFORM,
> regular virtio code won't work.
> - DMA API is giving us addresses that are actually also physical
> addresses.
> - Hypervisor has not enabled VIRTIO_F_ACCESS_PLATFORM.
>
> and if all conditions are true, we force all data through the bounce
> buffer.
>
> To put it another way, from hypervisor's point of view DMA API is
> not required: hypervisor would be happy to get access to all of guest
> memory. That's why it does not set VIRTIO_F_ACCESS_PLATFORM. However,
> guest decides that it does not trust the hypervisor and wants to force
> a bounce buffer for its own reasons.
>
>
> Thiago Jung Bauermann (2):
> dma-mapping: Add dma_addr_is_phys_addr()
> virtio_ring: Use DMA API if memory is encrypted
>
> arch/powerpc/include/asm/dma-mapping.h | 21 +++++++++++++++++++++
> arch/powerpc/platforms/pseries/Kconfig | 1 +
> drivers/virtio/virtio.c | 18 ++++++++++++++++++
> drivers/virtio/virtio_ring.c | 8 ++++++++
> include/linux/dma-mapping.h | 20 ++++++++++++++++++++
> include/linux/virtio_config.h | 14 ++++++++++++++
> kernel/dma/Kconfig | 3 +++
> 7 files changed, 85 insertions(+)
>
> --
> 1.8.3.1

--
Ram Pai

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