On Wed, Aug 27, 2014 at 10:35 AM, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote: > On Wed, Aug 27, 2014 at 09:29:36AM +0200, Christian Borntraeger wrote: >> On 26/08/14 23:17, Andy Lutomirski wrote: >> > virtio_ring currently sends the device (usually a hypervisor) >> > physical addresses of its I/O buffers. This is okay when DMA >> > addresses and physical addresses are the same thing, but this isn't >> > always the case. For example, this never works on Xen guests, and >> > it is likely to fail if a physical "virtio" device ever ends up >> > behind an IOMMU or swiotlb. >> > >> > The immediate use case for me is to enable virtio on Xen guests. >> > For that to work, we need this fix as well as a corresponding >> > fix to virtio_pci or to another driver. >> > >> > With this patch, virtfs survives kmemleak and CONFIG_DMA_API_DEBUG. >> > virtio-net warns (correctly) about DMA from the stack in >> > virtnet_set_rx_mode. >> > >> > This breaks s390's defconfig. The default configuration for s390 >> > does virtio through a KVM-specific interface, but that configuration >> > does not support DMA. I could modify this patch to stub out the DMA >> > API calls if !CONFIG_HAS_DMA, but it seems to me that it would be >> > much nicer to make s390 support DMA unconditionally. >> >> s390 has no DMA per se. Newest systems have a PCI-like I/O attach in >> addition to the classic channel I/O, and for that we enable the DMA code >> just for that transport to be able to reuse some of the existing PCI >> drivers. (only some because, we differ in some aspects from how PCI >> looks like) But the architecture itself (and the virtio interface) does >> not provide the DMA interface as you know it: > > Don't most of those DMA_API end up then being nops? As in, we do > have in the dma-api file the #ifdef case when a platform does not do > DMA which ends up with all functions stubbed out. > This doesn't work on s390. It fails to link. This was my initial thought, too -- it would be nice if s390 set up no-op DMA ops its virtio device. But this might be overkill, if virtio is the *only* device on s390 that does anything that looks like DMA. Anyway, v2 (coming soon) adds a switch in virtio, so s390 should continue to work. --Andy _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
