Re: [PATCH] xen: introduce xen_vring_use_dma

[Stefano Stabellini]

On Wed, 24 Jun 2020, Michael S. Tsirkin wrote:
> On Wed, Jun 24, 2020 at 10:59:47AM -0700, Stefano Stabellini wrote:
> > On Wed, 24 Jun 2020, Michael S. Tsirkin wrote:
> > > On Wed, Jun 24, 2020 at 05:17:32PM +0800, Peng Fan wrote:
> > > > Export xen_swiotlb for all platforms using xen swiotlb
> > > > 
> > > > Use xen_swiotlb to determine when vring should use dma APIs to map the
> > > > ring: when xen_swiotlb is enabled the dma API is required. When it is
> > > > disabled, it is not required.
> > > > 
> > > > Signed-off-by: Peng Fan <peng.fan@xxxxxxx>
> > > 
> > > Isn't there some way to use VIRTIO_F_IOMMU_PLATFORM for this?
> > > Xen was there first, but everyone else is using that now.
> > 
> > Unfortunately it is complicated and it is not related to
> > VIRTIO_F_IOMMU_PLATFORM :-(
> > 
> > 
> > The Xen subsystem in Linux uses dma_ops via swiotlb_xen to translate
> > foreign mappings (memory coming from other VMs) to physical addresses.
> > On x86, it also uses dma_ops to translate Linux's idea of a physical
> > address into a real physical address (this is unneeded on ARM.)
> > 
> > 
> > So regardless of VIRTIO_F_IOMMU_PLATFORM, dma_ops should be used on Xen/x86
> > always and on Xen/ARM if Linux is Dom0 (because it has foreign
> > mappings.) That is why we have the if (xen_domain) return true; in
> > vring_use_dma_api.
> 
> VIRTIO_F_IOMMU_PLATFORM makes guest always use DMA ops.
> 
> Xen hack predates VIRTIO_F_IOMMU_PLATFORM so it *also*
> forces DMA ops even if VIRTIO_F_IOMMU_PLATFORM is clear.
>
> Unfortunately as a result Xen never got around to
> properly setting VIRTIO_F_IOMMU_PLATFORM.

I don't think VIRTIO_F_IOMMU_PLATFORM would be correct for this because
the usage of swiotlb_xen is not a property of virtio, it is a detail of
the way Linux does Xen address translations. swiotlb-xen is used to do
these translations and it is hooked into the dma_ops framework.

It would be possible to have a device in hardware that is
virtio-compatible and doesn't set VIRTIO_F_IOMMU_PLATFORM. The device
could be directly assigned (passthrough) to a DomU. We would still
have to use swiotlb_xen if Xen is running.

You should think of swiotlb-xen as only internal to Linux and not
related to whether the (virtual or non-virtual) hardware comes with an
IOMMU or not.


> > You might have noticed that I missed one possible case above: Xen/ARM
> > DomU :-)
> > 
> > Xen/ARM domUs don't need swiotlb_xen, it is not even initialized. So if
> > (xen_domain) return true; would give the wrong answer in that case.
> > Linux would end up calling the "normal" dma_ops, not swiotlb-xen, and
> > the "normal" dma_ops fail.
> > 
> > 
> > The solution I suggested was to make the check in vring_use_dma_api more
> > flexible by returning true if the swiotlb_xen is supposed to be used,
> > not in general for all Xen domains, because that is what the check was
> > really meant to do.
> 
> Why not fix DMA ops so they DTRT (nop) on Xen/ARM DomU? What is wrong with that?

swiotlb-xen is not used on Xen/ARM DomU, the default dma_ops are the
ones that are used. So you are saying, why don't we fix the default
dma_ops to work with virtio?

It is bad that the default dma_ops crash with virtio, so yes I think it
would be good to fix that. However, even if we fixed that, the if
(xen_domain()) check in vring_use_dma_api is still a problem.


Alternatively we could try to work-around it from swiotlb-xen. We could
enable swiotlb-xen for Xen/ARM DomUs with a different implementation so
that we could leave the vring_use_dma_api check unmodified.

It would be ugly because we would have to figure out from the new
swiotlb-xen functions if the device is a normal device, so we have to
call the regular dma_ops functions, or if the device is a virtio device,
in which case there is nothing to do. I think it is undesirable but
could probably be made to work.