Re: [PATCH] xen: introduce xen_vring_use_dma

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jun 25, 2020 at 10:31:27AM -0700, Stefano Stabellini wrote:
> On Wed, 24 Jun 2020, Michael S. Tsirkin wrote:
> > On Wed, Jun 24, 2020 at 02:53:54PM -0700, Stefano Stabellini wrote:
> > > 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@nxp.com>
> > > > > > 
> > > > > > 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,
> > 
> > 
> > Basically any device without VIRTIO_F_ACCESS_PLATFORM
> > (that is it's name in latest virtio spec, VIRTIO_F_IOMMU_PLATFORM is
> > what linux calls it) is declared as "special, don't follow normal rules
> > for access".
> > 
> > So yes swiotlb_xen is not a property of virtio, but what *is* a property
> > of virtio is that it's not special, just a regular device from DMA POV.
> 
> I am trying to understand what you meant but I think I am missing
> something.
> 
> Are you saying that modern virtio should always have
> VIRTIO_F_ACCESS_PLATFORM, hence use normal dma_ops as any other devices?

I am saying it's a safe default. Clear VIRTIO_F_ACCESS_PLATFORM if you
have some special needs e.g. you are very sure it's ok to bypass DMA
ops, or you need to support a legacy guest (produced in the window
between virtio 1 support in 2014 and support for
VIRTIO_F_ACCESS_PLATFORM in 2016).


> If that is the case, how is it possible that virtio breaks on ARM using
> the default dma_ops? The breakage is not Xen related (except that Xen
> turns dma_ops on). The original message from Peng was:
> 
>   vring_map_one_sg -> vring_use_dma_api
>                    -> dma_map_page
>   		       -> __swiotlb_map_page
>   		                ->swiotlb_map_page
>   				->__dma_map_area(phys_to_virt(dma_to_phys(dev, dev_addr)), size, dir);
>   However we are using per device dma area for rpmsg, phys_to_virt
>   could not return a correct virtual address for virtual address in
>   vmalloc area. Then kernel panic.
> 
> I must be missing something. Maybe it is because it has to do with RPMesg?

I think it's an RPMesg bug, yes.

> 
> > > > > 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.
> > 
> > Why is it a problem? It just makes virtio use DMA API.
> > If that in turn works, problem solved.
> 
> You are correct in the sense that it would work. However I do think it
> is wrong for vring_use_dma_api to enable dma_ops/swiotlb-xen for Xen/ARM
> DomUs that don't need it. There are many different types of Xen guests,
> Xen x86 is drastically different from Xen ARM, it seems wrong to treat
> them the same way.

I could imagine some future Xen hosts setting a flag somewhere in the
platform capability saying "no xen specific flag, rely on
"VIRTIO_F_ACCESS_PLATFORM". Then you set that accordingly in QEMU.
How about that?


> 
> 
> Anyway, re-reading the last messages of the original thread [1], it
> looks like Peng had a clear idea on how to fix the general issue. Peng,
> what happened with that?
> 
> 
> [1] https://lore.kernel.org/patchwork/patch/1033801/#1222404