Re: [PATCH] arm/mm: add option to prefer IOMMU ops for DMA on Xen

[Stefano Stabellini]

On Tue, 14 Nov 2023, Robin Murphy wrote:
> On 11/11/2023 6:45 pm, Chuck Zmudzinski wrote:
> > Enabling the new option, ARM_DMA_USE_IOMMU_XEN, fixes this error when
> > attaching the Exynos mixer in Linux dom0 on Xen on the Chromebook Snow
> > (and probably on other devices that use the Exynos mixer):
> > 
> > [drm] Exynos DRM: using 14400000.fimd device for DMA mapping operations
> > exynos-drm exynos-drm: bound 14400000.fimd (ops 0xc0d96354)
> > exynos-mixer 14450000.mixer: [drm:exynos_drm_register_dma] *ERROR* Device
> >                               14450000.mixer lacks support for IOMMU
> > exynos-drm exynos-drm: failed to bind 14450000.mixer (ops 0xc0d97554): -22
> > exynos-drm exynos-drm: adev bind failed: -22
> > exynos-dp: probe of 145b0000.dp-controller failed with error -22
> > 
> > Linux normally uses xen_swiotlb_dma_ops for DMA for all devices when
> > xen_swiotlb is detected even when Xen exposes an IOMMU to Linux. Enabling
> > the new config option allows devices such as the Exynos mixer to use the
> > IOMMU instead of xen_swiotlb_dma_ops for DMA and this fixes the error.
> > 
> > The new config option is not set by default because it is likely some
> > devices that use IOMMU for DMA on Xen will cause DMA errors and memory
> > corruption when Xen PV block and network drivers are in use on the system.
> > 
> > Link:
> > https://lore.kernel.org/xen-devel/acfab1c5-eed1-4930-8c70-8681e256c820@xxxxxxxxxxxx/
> > 
> > Signed-off-by: Chuck Zmudzinski <brchuckz@xxxxxxx>
> > ---
> > The reported error with the Exynos mixer is not fixed by default by adding
> > a second patch to select the new option in the Kconfig definition for the
> > Exynos mixer if EXYNOS_IOMMU and SWIOTLB_XEN are enabled because it is
> > not certain setting the config option is suitable for all cases. So it is
> > necessary to explicitly select the new config option during the config
> > stage of the Linux kernel build to fix the reported error or similar
> > errors that have the same cause of lack of support for IOMMU on Xen. This
> > is necessary to avoid any regressions that might be caused by enabling the
> > new option by default for the Exynos mixer.
> >   arch/arm/mm/dma-mapping.c |  6 ++++++
> >   drivers/xen/Kconfig       | 16 ++++++++++++++++
> >   2 files changed, 22 insertions(+)
> > 
> > diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> > index 5409225b4abc..ca04fdf01be3 100644
> > --- a/arch/arm/mm/dma-mapping.c
> > +++ b/arch/arm/mm/dma-mapping.c
> > @@ -1779,6 +1779,12 @@ void arch_setup_dma_ops(struct device *dev, u64
> > dma_base, u64 size,
> >   	if (iommu)
> >   		arm_setup_iommu_dma_ops(dev, dma_base, size, iommu, coherent);
> >   +#ifdef CONFIG_ARM_DMA_USE_IOMMU_XEN
> 
> FWIW I don't think this really needs a config option - if Xen *has* made an
> IOMMU available, then there isn't really much reason not to use it, and if for
> some reason someone really didn't want to then they could simply disable the
> IOMMU driver anyway.

The fact that the Exynos IOMMU is exposed to Linux is a mistake. Xen
doesn't recognize the Exynos IOMMU (it is not one of the IOMMUs Xen has
a driver for) so it assigns the IOMMU to Dom0. It doesn't happen on
purpose, it happens by accident. Certain things are going to break,
specifically I am fairly certain PV drivers are going to break.

If Xen recognized the Exynos IOMMU as an IOMMU it would probably hide it
from Dom0. (Today Xen doesn't have a list of IOMMUs Xen recognizes but
doesn't have a driver for.)

I think it is OK for Chuck and others to play around with this
configuration but I wouldn't add a new kconfig option to Linux to
support it.

If we do want a kconfig option, I would add a kconfig option or Linux
command line option to enable/disable swiotlb-xen. Basically a way to
force-enable or force-disable xen_swiotlb_detect(). That could be
generally useful for debugging and would also solve the problem here as
it could be used to force-disable swiotlb-xen. I would imagine that the
end result is the same: the default ops (iommu_ops) are used.



> > +	if (dev->dma_ops == &iommu_ops) {
> > +		dev->archdata.dma_ops_setup = true;
> 
> The existing assignment is effectively unconditional by this point anyway, so
> could probably just be moved earlier to save duplicating it (or perhaps just
> make the xen_setup_dma_ops() call conditional instead to save the early return
> as well).
> 
> However, are the IOMMU DMA ops really compatible with Xen? The comments about
> hypercalls and foreign memory in xen_arch_need_swiotlb() leave me concerned
> that assuming non-coherent DMA to any old Dom0 page is OK might not actually
> work in general :/

Xen has (not yet upstreaming) support for nested IOMMU (Xen uses the
IOMMU while also it exposes a virtual IOMMU to guests.) In those cases
the iommu_ops should be compatible with Xen.

swiotlb-xen is useful in cases where there is no IOMMU on the platform
(or the IOMMU doesn't cover all DMA-capable devices) and Dom0 is 1:1
mapped. See include/xen/arm/swiotlb-xen.h:xen_swiotlb_detect. If Dom0 is
not 1:1 mapped swiotlb-xen doesn't work. If an IOMMU is present and
covers all DMA-capable devices, then swiotlb-xen is superfluous. This
last case is the interesting case for virtual IOMMU and Linux usage of
iommu_ops.