[RFCv2 PATCH 0/7] A General Accelerator Framework, WarpDrive

Kenneth Lee liguozhu at hisilicon.com
Fri Sep 21 20:03:14 AEST 2018

On Sun, Sep 16, 2018 at 09:42:44PM -0400, Jerome Glisse wrote:
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> Date: Sun, 16 Sep 2018 21:42:44 -0400
> From: Jerome Glisse <jglisse at redhat.com>
> To: Kenneth Lee <nek.in.cn at gmail.com>
> CC: Jonathan Corbet <corbet at lwn.net>, Herbert Xu
>  <herbert at gondor.apana.org.au>, "David S . Miller" <davem at davemloft.net>,
>  Joerg Roedel <joro at 8bytes.org>, Alex Williamson
>  <alex.williamson at redhat.com>, Kenneth Lee <liguozhu at hisilicon.com>, Hao
>  Fang <fanghao11 at huawei.com>, Zhou Wang <wangzhou1 at hisilicon.com>, Zaibo Xu
>  <xuzaibo at huawei.com>, Philippe Ombredanne <pombredanne at nexb.com>, Greg
>  Kroah-Hartman <gregkh at linuxfoundation.org>, Thomas Gleixner
>  <tglx at linutronix.de>, linux-doc at vger.kernel.org,
>  linux-kernel at vger.kernel.org, linux-crypto at vger.kernel.org,
>  iommu at lists.linux-foundation.org, kvm at vger.kernel.org,
>  linux-accelerators at lists.ozlabs.org, Lu Baolu <baolu.lu at linux.intel.com>,
>  Sanjay Kumar <sanjay.k.kumar at intel.com>, linuxarm at huawei.com
> Subject: Re: [RFCv2 PATCH 0/7] A General Accelerator Framework, WarpDrive
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> So i want to summarize issues i have as this threads have dig deep into
> details. For this i would like to differentiate two cases first the easy
> one when relying on SVA/SVM. Then the second one when there is no SVA/SVM.
> In both cases your objectives as i understand them:
> [R1]- expose a common user space API that make it easy to share boiler
>       plate code accross many devices (discovering devices, opening
>       device, creating context, creating command queue ...).
> [R2]- try to share the device as much as possible up to device limits
>       (number of independant queues the device has)
> [R3]- minimize syscall by allowing user space to directly schedule on the
>       device queue without a round trip to the kernel
> I don't think i missed any.
> (1) Device with SVA/SVM
> For that case it is easy, you do not need to be in VFIO or part of any
> thing specific in the kernel. There is no security risk (modulo bug in
> the SVA/SVM silicon). Fork/exec is properly handle and binding a process
> to a device is just couple dozen lines of code.
> (2) Device does not have SVA/SVM (or it is disabled)
> You want to still allow device to be part of your framework. However
> here i see fundamentals securities issues and you move the burden of
> being careful to user space which i think is a bad idea. We should
> never trus the userspace from kernel space.
> To keep the same API for the user space code you want a 1:1 mapping
> between device physical address and process virtual address (ie if
> device access device physical address A it is accessing the same
> memory as what is backing the virtual address A in the process.
> Security issues are on two things:
> [I1]- fork/exec, a process who opened any such device and created an
>       active queue can transfer without its knowledge control of its
>       commands queue through COW. The parent map some anonymous region
>       to the device as a command queue buffer but because of COW the
>       parent can be the first to copy on write and thus the child can
>       inherit the original pages that are mapped to the hardware.
>       Here parent lose control and child gain it.

Hi, Jerome, 

I reconsider your logic. I think the problem can be solved. Let us separate the
SVA/SVM feature into two: fault-from-device and device-va-awareness. A device
with iommu can support only device-va-awareness or both.

VFIO works on top of iommu, so it will support at least device-va-awareness. For
the COW problem, it can be taken as a mmu synchronization issue. If the mmu page
table is changed, it should be synchronize to iommu (via iommu_notifier). In the
case that the device support fault-from-device, it will work fine. In the case
that it supports only device-va-awareness, we can prefault (handle_mm_fault)
also via iommu_notifier and reset to iommu page table.

So this can be considered as a bug of VFIO, cannot it?

> [I2]- Because of [R3] you want to allow userspace to schedule commands
>       on the device without doing an ioctl and thus here user space
>       can schedule any commands to the device with any address. What
>       happens if that address have not been mapped by the user space
>       is undefined and in fact can not be defined as what each IOMMU
>       does on invalid address access is different from IOMMU to IOMMU.
>       In case of a bad IOMMU, or simply an IOMMU improperly setup by
>       the kernel, this can potentialy allow user space to DMA anywhere.
> [I3]- By relying on GUP in VFIO you are not abiding by the implicit
>       contract (at least i hope it is implicit) that you should not
>       try to map to the device any file backed vma (private or share).
>       The VFIO code never check the vma controlling the addresses that
>       are provided to VFIO_IOMMU_MAP_DMA ioctl. Which means that the
>       user space can provide file backed range.
>       I am guessing that the VFIO code never had any issues because its
>       number one user is QEMU and QEMU never does that (and that's good
>       as no one should ever do that).
>       So if process does that you are opening your self to serious file
>       system corruption (depending on file system this can lead to total
>       data loss for the filesystem).
>       Issue is that once you GUP you never abide to file system flushing
>       which write protect the page before writing to the disk. So
>       because the page is still map with write permission to the device
>       (assuming VFIO_IOMMU_MAP_DMA was a write map) then the device can
>       write to the page while it is in the middle of being written back
>       to disk. Consult your nearest file system specialist to ask him
>       how bad that can be.

In the case, we cannot do anything if the device do not support
fault-from-device. But we can reject write map with file-backed mapping.

It seems both issues can be solved under VFIO framework:) (But of cause, I don't
mean it has to)

> [I4]- Design issue, mdev design As Far As I Understand It is about
>       sharing a single device to multiple clients (most obvious case
>       here is again QEMU guest). But you are going against that model,
>       in fact AFAIUI you are doing the exect opposite. When there is
>       no SVA/SVM you want only one mdev device that can not be share.
>       So this is counter intuitive to the mdev existing design. It is
>       not about sharing device among multiple users but about giving
>       exclusive access to the device to one user.
> All the reasons above is why i believe a different model would serve
> you and your user better. Below is a design that avoids all of the
> above issues and still delivers all of your objectives with the
> exceptions of the third one [R3] when there is no SVA/SVM.
> Create a subsystem (very much boiler plate code) which allow device to
> register themself against (very much like what you do in your current
> patchset but outside of VFIO).
> That subsystem will create a device file for each registered system and
> expose a common API (ie set of ioctl) for each of those device files.
> When user space create a queue (through an ioctl after opening the device
> file) the kernel can return -EBUSY if all the device queue are in use,
> or create a device queue and return a flag like SYNC_ONLY for device that
> do not have SVA/SVM.
> For device with SVA/SVM at the time the process create a queue you bind
> the process PASID to the device queue. From there on the userspace can
> schedule commands and use the device without going to kernel space.
> For device without SVA/SVM you create a fake queue that is just pure
> memory is not related to the device. From there on the userspace must
> call an ioctl every time it wants the device to consume its queue
> (hence why the SYNC_ONLY flag for synchronous operation only). The
> kernel portion read the fake queue expose to user space and copy
> commands into the real hardware queue but first it properly map any
> of the process memory needed for those commands to the device and
> adjust the device physical address with the one it gets from dma_map
> API.
> With that model it is "easy" to listen to mmu_notifier and to abide by
> them to avoid issues [I1], [I3] and [I4]. You obviously avoid the [I2]
> issue by only mapping a fake device queue to userspace.
> So yes with that models it means that every device that wish to support
> the non SVA/SVM case will have to do extra work (ie emulate its command
> queue in software in the kernel). But by doing so, you support an
> unlimited number of process on your device (ie all the process can share
> one single hardware command queues or multiple hardware queues).
> The big advantages i see here is that the process do not have to worry
> about doing something wrong. You are protecting yourself and your user
> from stupid mistakes.
> I hope this is useful to you.
> Cheers,
> Jérôme


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