On Thu, Aug 02, 2018 at 12:53:41PM -0500, Benjamin Herrenschmidt wrote: > On Thu, 2018-08-02 at 20:19 +0300, Michael S. Tsirkin wrote: > > > > I see. So yes, given that device does not know or care, using > > virtio features is an awkward fit. > > > > So let's say as a quick fix for you maybe we could generalize the > > xen_domain hack, instead of just checking xen_domain check some static > > branch. Then teach xen and others to enable that.> > > > OK but problem then becomes this: if you do this and virtio device appears > > behind a vIOMMU and it does not advertize the IOMMU flag, the > > code will try to use the vIOMMU mappings and fail. > > > > It does look like even with trick above, you need a special version of > > DMA ops that does just swiotlb but not any of the other things DMA API > > might do. > > > > Thoughts? > > Yes, this is the purpose of Anshuman original patch (I haven't looked > at the details of the patch in a while but that's what I told him to > implement ;-) : > > - Make virtio always use DMA ops to simplify the code path (with a set > of "transparent" ops for legacy) > > and > > - Provide an arch hook allowing us to "override" those "transparent" > DMA ops with some custom ones that do the appropriate swiotlb gunk. > > Cheers, > Ben. > Right but as I tried to say doing that brings us to a bunch of issues with using DMA APIs in virtio. Put simply DMA APIs weren't designed for guest to hypervisor communication. When we do (as is the case with PLATFORM_IOMMU right now) this adds a bunch of overhead which we need to get rid of if we are to switch to PLATFORM_IOMMU by default. We need to fix that. -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
