Re: RFC: use VFIO over a UNIX domain socket to implement device offloading

[Stefan Hajnoczi]

On Thu, Jun 25, 2020 at 08:54:25PM -0700, John G Johnson wrote:
> 
> 
> > On Jun 23, 2020, at 5:27 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > 
> > On Thu, Jun 18, 2020 at 02:38:04PM -0700, John G Johnson wrote:
> >>> On Jun 15, 2020, at 3:49 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >>> An issue with file descriptor passing is that it's hard to revoke access
> >>> once the file descriptor has been passed. memfd supports sealing with
> >>> fnctl(F_ADD_SEALS) it doesn't revoke mmap(MAP_WRITE) on other processes.
> >>> 
> >>> Memory Protection Keys don't seem to be useful here either and their
> >>> availability is limited (see pkeys(7)).
> >>> 
> >>> One crazy idea is to use KVM as a sandbox for running the device and let
> >>> the vIOMMU control the page tables instead of the device (guest). That
> >>> way the hardware MMU provides memory translation, but I think this is
> >>> impractical because the guest environment is too different from the
> >>> Linux userspace environment.
> >>> 
> >>> As a starting point adding DMA_READ/DMA_WRITE messages would provide the
> >>> functionality and security. Unfortunately it makes DMA expensive and
> >>> performance will suffer.
> >>> 
> >> 
> >>    Are you advocating for only using VFIO_USER_DMA_READ/WRITE and
> >> not passing FDs at all?  The performance penalty would be large for the
> >> cases where the client and server are equally trusted.  Or are you
> >> advocating for an option where the slower methods are used for cases
> >> where the server is less trusted?
> > 
> > I think the enforcing IOMMU should be optional (due to the performance
> > overhead) but part of the spec from the start.
> > 
> 
> 
>       With this in mind, we will collapse the current memory region
> messages (VFIO_USER_ADD_MEMORY_REGION and VFIO_USER_SUB_MEMORY_REGION)
> and the IOMMU messages (VFIO_USER_IOMMU_MAP and VFIO_USER_IOMMU_UNMAP)
> into new messages (VFIO_USER_DMA_MAP and VFIO_USER_DMA_UNMAP).  Their
> contents will be the same as the memory region messages.
> 
>       On a system without an IOMMU, the new messages will be used to
> export the system physical address space as DMA addresses.  On a system
> with an IOMMU they will be used to export the valid device DMA ranges
> programmed into the IOMMU by the guest.  This behavior matches how the
> existing QEMU VFIO object programs the host IOMMU.  The server will not
> be aware of whether the client is using an IOMMU.
>
>       In the QEMU VFIO implementation, will will add a ‘secure-dma’
> option that suppresses exporting mmap()able FDs to the server.  All
> DMA will use the slow path to be validated by the client before accessing
> guest memory.
> 
>       Is this acceptable to you (and Alex, of course)?

Sounds good to me.

Stefan

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