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SubjectRe: [RFC 0/3] iommu: Reserved regions for IOVAs beyond dma_mask and iommu aperture
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Hi Alex,

On 9/29/20 8:18 PM, 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 [also correcting some outdated email addresses + adding Lorenzo in cc]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.
OK
>
>> 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 see it either. So the dma_mask set by the driver would never be
checked against the dma_mask limited found when parsing OF/ACPI?
>
>> In my case the 42b limit is computed in iort_dma_setup() by
>> acpi_dma_get_range(dev, &dmaaddr, &offset, &size);
>>
>> Referring to the comment, it does "Evaluate DMA regions and return
>> respectively DMA region start, offset and size in dma_addr, offset and
>> size on parsing success". This parses the ACPI table, looking for ACPI
>> companions with _DMA methods.
>>
>> But as Alex mentioned, the IORT also allows to define limits on "the
>> number of address bits, starting from the least significant bit that can
>> be generated by a device when it accesses memory". See Named component
>> node.Device Memory Address Size limit or PCI root complex node. Memory
>> address size limit.
>>
>> ret = acpi_dma_get_range(dev, &dmaaddr, &offset, &size);
>> if (ret == -ENODEV)
>> ret = dev_is_pci(dev) ? rc_dma_get_range(dev, &size)
>> : nc_dma_get_range(dev, &size);
>>
>> So eventually those info collected from the ACPI tables which do impact
>> the usable IOVA range seem to be stored in the dma_mask, hence that
>> proposal.
>
> As above, it's not clear to me that anyone other than the driver and
> the dma_supported() callback on dma_ops have any input on the value of
> dma_mask, so I'm a little baffled by the current operation.

me too
>
>>>> So, for instance, if the IOMMU supports up to 48bits, it may give
>>>> the impression the max IOVA is 48b while a device may have a
>>>> dma_mask of 42b. So this API cannot really be used to compute
>>>> the max usable IOVA.
>>>>
>>>> This patch removes the IOVA region beyond the dma_mask's.
>>>
>>> Rather it adds a reserved region accounting for the range above the
>>> device's dma_mask.
>>
>> Yep. It adds new reserved regions in
>> /sys/kernel/iommu_groups/<n>/reserved_regions and remove those from the
>> usable regions exposed by VFIO GET_INFO.
>>
>> I don't think the IOMMU API should be consuming
>>> dma_mask like this though. For example, what happens in
>>> pci_dma_configure() when there are no OF or ACPI DMA restrictions?
>> My guess was that the dma_mask was set to the max range but I did not
>> test it.
>
> Still, we're making use of a driver configured property for the
> purposes of using the DMA API and consuming it in the IOMMU API,
> specifically to satisfy a userspace driver where the in-kernel meta-
> driver can't make any assumptions about the device DMA mask. It's all
> rather convoluted.

I can't object here. Still we are missing missing consolidated info
about max IOVA range. I interpreted the dma_mask as the info missing to
get it consolidated but now we are not even sure it does the job and
even I would hijack its original goal.
>> It
>>> appears to me that the dma_mask from whatever previous driver had the
>>> device carries over to the new driver. That's generally ok for the DMA
>>> API because a driver is required to set the device's DMA mask. It
>>> doesn't make sense however to blindly consume that dma_mask and export
>>> it via an IOMMU API. For example I would expect to see different
>>> results depending on whether a host driver has been bound to a device.
>>> It seems the correct IOMMU API approach would be for the IORT code to
>>> specifically register reserved ranges for the device.
>>
>> Is it only specific to IORT table? acpi_dma_get_range() in
>> drivers/acpi/scan.c is generic.
>
> Yes, anything trying to implement similar restrictions. It appears to
> me that platform code is stepping on a driver owned field used by
> dma_ops already here. Maybe reserved regions should be consumed by
> dma_ops to understand restrictions between the device and the IOMMU.
>
>>>> As we start to expose this reserved region in the sysfs file
>>>> /sys/kernel/iommu_groups/<n>/reserved_regions, we also need to
>>>> handle the IOVA range beyond the IOMMU aperture to handle the case
>>>> where the dma_mask would have a higher number of bits than the iommu
>>>> max input address.
>>>
>>> Why? The IOMMU geometry already describes this and vfio combines both
>>> the IOMMU geometry and the device reserved regions when generating the
>>> IOVA ranges?
>> Yes VFIO layer does add the info about the topology but
>> /sys/kernel/iommu_groups/<n>/reserved_regions, generated by the IOMMU
>> code, does not. this latter only exposes reserved regions. Assume the
>> dma_mask is 48b and the IOMMU aperture is 42b (assuming it is possible),
>> if you only take into account the "dma_mask" limitation, the end-user
>> will interpret this as: I can use up to 48b.
>
> What end user?
Anyone who reads /sys/kernel/iommu_groups/<n>/reserved_regions. A user
application could use that info. Anyway I think I indeed wants this file
to do more than it was originally meant for, enumerating *some* of the
reserved regions. That's a bit frustrating though (same for the VFIO ioctl).

The DMA API is allocating within the address space of
> the IOMMU, so it will simply never encounter the issue. Within the
> IOMMU API we can already query the geometry of the IOMMU to know its
> width. It seems like reserved regions is trying to take on new
> responsibilities here.
>
>> Who is going to consume this information? Additionally
>>> it appears that reserved regions will report different information
>>> depending on whether a device is attached to a domain.
>> yes that's correct. Well at some point we decided to expose (some)
>> reserved regions through sysfs. Only printing a reduced set of those
>> also can be misleading, hence my attempt to be more comprehensive.
>
> Seems it further blurs the lines of what reserved regions is intended
> to report. Is it trying to replace the IOMMU API geometry interface?
>
>>>> This is a change to the ABI as this reserved region was not yet
>>>> exposed in sysfs /sys/kernel/iommu_groups/<n>/reserved_regions or
>>>> through the VFIO ioctl. At VFIO level we increment the version of
>>>> the VFIO_IOMMU_TYPE1_INFO_CAP_IOVA_RANGE capability to advertise
>>>> that change.
>>>
>>> Is this really an ABI change? The original entry for reserved regions
>>> includes:
>>>
>>> Not necessarily all reserved regions are listed. This is typically
>>> used to output direct-mapped, MSI, non mappable regions.
>>
>> I agree. That's not really a change in the ABI but I wanted to make
>> things clear about the induced changes for the end-user. On the other
>> end there will be a change in the number of reported resv regions.
>>>
>>> I imagine the intention here was non-mappable relative to the IOMMU,
>>> but non-mappable to the device is essentially what we're including
>>> here.
>>>
>>> I'm also concerned about bumping the vfio interface version for the
>>> IOVA range. We're not changing the interface, we're modifying the
>>> result, and even then only for a fraction of users. How many users are
>>> potentially broken by that change? Are we going to bump the version
>>> for everyone any time the result changes on any platform? Thanks,
>>
>> The userspace needs to know if the GET_INFO is reliable to compute the
>> min/max IOVAs. If we do not change the version, it cannot know and must
>> assume it is not. Wasn't the version field meant for that somehow?
>
> Our IOVA RANGE is correct to the best of our knowledge, but I don't
> think we can or should rev the version every time we find something
> incorrect. For instance, we wouldn't do the same for a firmware
> induced range that was missing or incorrect. I think the version field
> is for creating a new data structure to replace or expand the existing
> one, not to simply say the returned data might be different. I
> understand the userspace dilemma, but this doesn't seem like the right
> solution. Thanks,
understood

Thanks

Eric
>
> Alex
>

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