Re: [RFC 0/4] Virtio uses DMA API for all devices

From: Christoph Hellwig
Date: Tue Aug 07 2018 - 02:18:22 EST

On Tue, Aug 07, 2018 at 12:46:34AM +0300, Michael S. Tsirkin wrote:
> Well we have the RFC for that - the switch to using DMA ops unconditionally isn't
> problematic itself IMHO, for now that RFC is blocked
> by its perfromance overhead for now but Christoph says
> he's trying to remove that for direct mappings,
> so we should hopefully be able to get there in X weeks.

The direct calls to dma_direct_ops aren't going to help you with legacy
virtio, given that virtio is specified to deal with physical addresses,
while dma-direct is not in many cases.

It would however help with the case where qemu always sets the platform
dma flag, as we'd avoid the indirect calls for that.