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Subject: Re: [RFC 0/3] virtio-iommu: a paravirtualized IOMMU
On 13/04/17 09:41, Tian, Kevin wrote: >> From: Jean-Philippe Brucker >> Sent: Saturday, April 8, 2017 3:18 AM >> >> This is the initial proposal for a paravirtualized IOMMU device using >> virtio transport. It contains a description of the device, a Linux driver, >> and a toy implementation in kvmtool. With this prototype, you can >> translate DMA to guest memory from emulated (virtio), or passed-through >> (VFIO) devices. >> >> In its simplest form, implemented here, the device handles map/unmap >> requests from the guest. Future extensions proposed in "RFC 3/3" should >> allow to bind page tables to devices. >> >> There are a number of advantages in a paravirtualized IOMMU over a full >> emulation. It is portable and could be reused on different architectures. >> It is easier to implement than a full emulation, with less state tracking. >> It might be more efficient in some cases, with less context switches to >> the host and the possibility of in-kernel emulation. >> >> When designing it and writing the kvmtool device, I considered two main >> scenarios, illustrated below. >> >> Scenario 1: a hardware device passed through twice via VFIO >> >> MEM____pIOMMU________PCI device________________________ >> HARDWARE >> | (2b) \ >> ----------|-------------+-------------+------------------\------------- >> | : KVM : \ >> | : : \ >> pIOMMU drv : _______virtio-iommu drv \ KERNEL >> | : | : | \ >> VFIO : | : VFIO \ >> | : | : | \ >> | : | : | / >> ----------|-------------+--------|----+----------|------------/-------- >> | | : | / >> | (1c) (1b) | : (1a) | / (2a) >> | | : | / >> | | : | / USERSPACE >> |___virtio-iommu dev___| : net drv___/ >> : >> --------------------------------------+-------------------------------- >> HOST : GUEST >> > > Usually people draw such layers in reverse order, e.g. hw in the > bottom then kernel in the middle then user in the top. :-) Alright, I'll keep that in mind. >> (1) a. Guest userspace is running a net driver (e.g. DPDK). It allocates a >> buffer with mmap, obtaining virtual address VA. It then send a >> VFIO_IOMMU_MAP_DMA request to map VA to an IOVA (possibly >> VA=IOVA). >> b. The maping request is relayed to the host through virtio >> (VIRTIO_IOMMU_T_MAP). >> c. The mapping request is relayed to the physical IOMMU through VFIO. >> >> (2) a. The guest userspace driver can now instruct the device to directly >> access the buffer at IOVA >> b. IOVA accesses from the device are translated into physical >> addresses by the IOMMU. >> >> Scenario 2: a virtual net device behind a virtual IOMMU. >> >> MEM__pIOMMU___PCI device HARDWARE >> | | >> -------|---------|------+-------------+------------------------------- >> | | : KVM : >> | | : : >> pIOMMU drv | : : >> \ | : _____________virtio-net drv KERNEL >> \_net drv : | : / (1a) >> | : | : / >> tap : | ________virtio-iommu drv >> | : | | : (1b) >> -----------------|------+-----|---|---+------------------------------- >> | | | : >> |_virtio-net_| | : >> / (2) | : >> / | : USERSPACE >> virtio-iommu dev______| : >> : >> --------------------------------------+------------------------------- >> HOST : GUEST >> >> (1) a. Guest virtio-net driver maps the virtio ring and a buffer >> b. The mapping requests are relayed to the host through virtio. >> (2) The virtio-net device now needs to access any guest memory via the >> IOMMU. >> >> Physical and virtual IOMMUs are completely dissociated. The net driver is >> mapping its own buffers via DMA/IOMMU API, and buffers are copied >> between >> virtio-net and tap. >> >> >> The description itself seemed too long for a single email, so I split it >> into three documents, and will attach Linux and kvmtool patches to this >> email. >> >> 1. Firmware note, >> 2. device operations (draft for the virtio specification), >> 3. future work/possible improvements. >> >> Just to be clear on the terms I'm using: >> >> pIOMMU physical IOMMU, controlling DMA accesses from physical >> devices >> vIOMMU virtual IOMMU (virtio-iommu), controlling DMA accesses >> from >> physical and virtual devices to guest memory. > > maybe clearer to call controlling 'virtual' DMA access since we're > essentially doing DMA virtualization here. Otherwise I read it > a bit confusing since DMA accesses from physical device should > be controlled by pIOMMU. > >> GVA, GPA, HVA, HPA >> Guest/Host Virtual/Physical Address >> IOVA I/O Virtual Address, the address accessed by a device doing DMA >> through an IOMMU. In the context of a guest OS, IOVA is GVA. > > This statement is not accurate. For kernel DMA protection, it is > per-device standalone address space (definitely nothing to do > with GVA). For user DMA protection, user space driver decides > how it wants to construct IOVA address space. could be a > standalone one, or reuse GVA. In virtualization case it is either > GPA (w/o vIOMMU) or guest IOVA (w/ IOMMU and guest creates > IOVA space). > > anyway IOVA concept is clear. possibly just removing the example > is still clear. :-) Ok, I dropped most IOVA references from the RFC to avoid ambiguity anyway. I'll tidy up my so-called clarifications next time :) Thanks, Jean-Philippe >> >> Note: kvmtool is GPLv2. Linux patches are GPLv2, except for UAPI >> virtio-iommu.h header, which is BSD 3-clause. For the time being, the >> specification draft in RFC 2/3 is also BSD 3-clause. >> >> >> This proposal may be involuntarily centered around ARM architectures at >> times. Any feedback would be appreciated, especially regarding other >> IOMMU >> architectures. >> > > thanks for doing this. will definitely look them in detail and feedback. > > Thanks > Kevin > >
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