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

From: Michael S. Tsirkin
Date: Thu Aug 02 2018 - 16:52:21 EST


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