Re: [RFC 0/3] iommu: Reserved regions for IOVAs beyond dma_mask and iommu aperture

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

[+Christoph]

On Tue, Sep 29, 2020 at 12:18:49PM -0600, Alex Williamson wrote:
> On Tue, 29 Sep 2020 09:18:22 +0200
> Auger Eric <eric.auger@redhat.com> wrote:
> 
> > Hi all,
> > 
> > [also correcting some outdated email addresses + adding Lorenzo in cc]
> > 
> > On 9/29/20 12:42 AM, Alex Williamson wrote:
> > > On Mon, 28 Sep 2020 21:50:34 +0200
> > > Eric Auger <eric.auger@redhat.com> wrote:
> > >   
> > >> VFIO currently exposes the usable IOVA regions through the
> > >> VFIO_IOMMU_GET_INFO ioctl / VFIO_IOMMU_TYPE1_INFO_CAP_IOVA_RANGE
> > >> capability. However it fails to take into account the dma_mask
> > >> of the devices within the container. The top limit currently is
> > >> defined by the iommu aperture.  
> > > 
> > > I think that dma_mask is traditionally a DMA API interface for a device
> > > driver to indicate to the DMA layer which mappings are accessible to the
> > > device.  On the other hand, vfio makes use of the IOMMU API where the
> > > driver is in userspace.  That userspace driver has full control of the
> > > IOVA range of the device, therefore dma_mask is mostly irrelevant to
> > > vfio.  I think the issue you're trying to tackle is that the IORT code
> > > is making use of the dma_mask to try to describe a DMA address
> > > limitation imposed by the PCI root bus, living between the endpoint
> > > device and the IOMMU.  Therefore, if the IORT code is exposing a
> > > topology or system imposed device limitation, this seems much more akin
> > > to something like an MSI reserved range, where it's not necessarily the
> > > device or the IOMMU with the limitation, but something that sits
> > > between them.  
> > 
> > First I think I failed to explain the context. I worked on NVMe
> > passthrough on ARM. The QEMU NVMe backend uses VFIO to program the
> > physical device. The IOVA allocator there currently uses an IOVA range
> > within [0x10000, 1ULL << 39]. This IOVA layout rather is arbitrary if I
> > understand correctly.
> 
> 39 bits is the minimum available on some VT-d systems, so it was
> probably considered a reasonable minimum address width to consider.
> 
> > I noticed we rapidly get some VFIO MAP DMA
> > failures because the allocated IOVA collide with the ARM MSI reserved
> > IOVA window [0x8000000, 0x8100000]. Since  9b77e5c79840 ("vfio/type1:
> > Check reserved region conflict and update iova list"), such VFIO MAP DMA
> > attempts to map IOVAs belonging to host reserved IOVA windows fail. So,
> > by using the VFIO_IOMMU_GET_INFO ioctl /
> > VFIO_IOMMU_TYPE1_INFO_CAP_IOVA_RANGE I can change the IOVA allocator to
> > avoid allocating within this range and others. While working on this, I
> > tried to automatically compute the min/max IOVAs and change the
> > arbitrary [0x10000, 1ULL << 39]. My SMMUv2 supports up to 48b so
> > naturally the max IOVA was computed as 1ULL << 48. The QEMU NVMe backend
> > allocates at the bottom and at the top of the range. I noticed the use
> > case was not working as soon as the top IOVA was more than 1ULL << 42.
> > And then we noticed the dma_mask was set to 42 by using
> > cat  /sys/bus/pci/devices/0005:01:00.0/dma_mask_bits. So my
> > interpretation is the dma_mask was somehow containing the info the
> > device couldn't handle IOVAs beyond a certain limit.
> 
> I see that there are both OF and ACPI hooks in pci_dma_configure() and
> both modify dev->dma_mask, which is what pci-sysfs is exposing here,
> but I'm not convinced this even does what it's intended to do.  The
> driver core calls this via the bus->dma_configure callback before
> probing a driver, but then what happens when the driver calls
> pci_set_dma_mask()?  This is just a wrapper for dma_set_mask() and I
> don't see anywhere that would take into account the existing
> dev->dma_mask.  It seems for example that pci_dma_configure() could
> produce a 42 bit mask as we have here, then the driver could override
> that with anything that the dma_ops.dma_supported() callback finds
> acceptable, and I don't see any instances where the current
> dev->dma_mask is considered.  Am I overlooking something? 

I don't think so but Christoph and Robin can provide more input on
this - it is a long story.

ACPI and OF bindings set a default dma_mask (and dev->bus_dma_limit),
this does not prevent a driver from overriding the dev->dma_mask but DMA
mapping code still takes into account the dev->bus_dma_limit.

This may help:

git log -p 03bfdc31176c

Thanks,
Lorenzo
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