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
