Revert "vring: Force use of DMA API for ARM-based systems with legacy devices"
This reverts commit c7070619. This has been shown to regress on some ARM systems: by forcing on DMA API usage for ARM systems, we have inadvertently kicked open a hornets' nest in terms of cache-coherency. Namely that unless the virtio device is explicitly described as capable of coherent DMA by firmware, the DMA APIs on ARM and other DT-based platforms will assume it is non-coherent. This turns out to cause a big problem for the likes of QEMU and kvmtool, which generate virtio-mmio devices in their guest DTs but neglect to add the often-overlooked "dma-coherent" property; as a result, we end up with the guest making non-cacheable accesses to the vring, the host doing so cacheably, both talking past each other and things going horribly wrong. We are working on a safer work-around. Fixes: c7070619 ("vring: Force use of DMA API for ARM-based systems with legacy devices") Reported-by: Robin Murphy <firstname.lastname@example.org> Cc: <email@example.com> Signed-off-by: Will Deacon <firstname.lastname@example.org> Signed-off-by: Michael S. Tsirkin <email@example.com> Acked-by: Marc Zyngier <firstname.lastname@example.org>
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