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

[Chuck Zmudzinski <brchuckz@xxxxxxxxxxxx>]

On 11/14/2023 5:20 PM, Stefano Stabellini wrote:
> 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.

I will try this. It isn't exactly what I have tested until now because
in all my tests so far all the DMA capable devices on the Chromebook use
swioltlb-xen except for the two devices that need to use the Exynos IOMMU
to fix the error with the Exynos mixer.

> 
> 
> 
>> > +  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.

It seems that swiotlb-xen works on this Chromebook since all but two
of the DMA capable devices use it when configured with the Kconfig option
added here and it seems to work fine so I presume Dom0 is 1:1 mapped as
expected. It is possible that on this device, the IOMMU is only covering
the two devices that need to use the Exynos IOMMU in the tests I have done.
There are many other DMA capable devices that use swiotlb-xen DMA ops
on Xen, but I have not checked what DMA ops the other devices use when
Linux runs on the Chromebook on bare metal without Xen.

So I plan to do some tests and see what DMA ops the other devices use if
swiotlb-xen is disabled and also what DMA ops the other devices use when
Linux runs on the Chromebook on bare metal without Xen. If these tests
show the problem can be fixed by disabling swiotlb-xen with a Kconfig  or
command line option, I will propose v2 to implement that as a solution.

> This last case is the interesting case for virtual IOMMU and Linux usage of
> iommu_ops.