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Subject: Re: [virtio-dev] Re: Constraining where a guest may allocate virtio accessible resources
On Thu, Jun 18, 2020 at 05:22:40PM +0200, Jan Kiszka wrote: > On 18.06.20 17:05, Michael S. Tsirkin wrote: > > On Thu, Jun 18, 2020 at 04:58:40PM +0200, Jan Kiszka wrote: > >>>>>>> Option 5 - Additional Device > >>>>>>> ============================ > >>>>>>> > >>>>>>> The final approach would be to tie the allocation of virtqueues to > >>>>>>> memory regions as defined by additional devices. For example the > >>>>>>> proposed IVSHMEMv2 spec offers the ability for the hypervisor to present > >>>>>>> a fixed non-mappable region of the address space. Other proposals like > >>>>>>> virtio-mem allow for hot plugging of "physical" memory into the guest > >>>>>>> (conveniently treatable as separate shareable memory objects for QEMU > >>>>>>> ;-). > >>>>>>> > >>>>>> > >>>>>> I think you forgot one approach: virtual IOMMU. That is the advanced > >>>>>> form of the grant table approach. The backend still "sees" the full > >>>>>> address space of the frontend, but it will not be able to access all of > >>>>>> it and there might even be a translation going on. Well, like IOMMUs work. > >>>>>> > >>>>>> However, this implies dynamics that are under guest control, namely of > >>>>>> the frontend guest. And such dynamics can be counterproductive for > >>>>>> certain scenarios. That's where this static windows of shared memory > >>>>>> came up. > >>>>> > >>>>> Yes, I think IOMMU interfaces are worth investigating more too. IOMMUs > >>>>> are now widely implemented in Linux and virtualization software. That > >>>>> means guest modifications aren't necessary and unmodified guest > >>>>> applications will run. > >>>>> > >>>>> Applications that need the best performance can use a static mapping > >>>>> while applications that want the strongest isolation can map/unmap DMA > >>>>> buffers dynamically. > >>>> > >>>> I do not see yet that you can model with an IOMMU a static, not guest > >>>> controlled window. > >>> > >>> Well basically the IOMMU will have as part of the > >>> topology description and range of addresses devices behind it > >>> are allowed to access. What's the problem with that? > >>> > >> > >> I didn't look at the detail of the vIOMMU from that perspective, but our > >> requirement would be that it would just statically communicate to the > >> guest where DMA windows are, rather than allowing the guest to configure > >> that (which is the normal usage of an IOMMU). > > > > Right, I got that - IOMMUs aren't necessarily fully configurable though. > > E.g. some IOMMUs are restricted in the # of bits they can address. > > > > > >> In addition, it would only address the memory transfer topic. We would > >> still be left with the current issue of virtio that the hypervisor's > >> device model needs to understand all supported device types. > >> > >> Jan > > > > I'd expect the DMA API would try to paper over that likely using > > bounce buffering. If you want to avoid copies, that's a harder > > problem generally. > > > > Here I was referring to the permutations of the control path in a device > model when switching from, say, a storage to a network virtio device. > With PCI and MMIO (didn't check Channel I/O, but that's not portable > anyway), you need to patch the "first-level" hypervisor when you want to > add a brand-new virtio-sound device and the hypervisor is not yet aware > of it. For minimized setups, I would prefer to only reconfigure it and > just add a new backend service app or VM. Naturally, that model also > shrinks the logic the core hypervisor needs to provide for virtio. > > Jan Hmm that went woosh over my head a bit, sorry. If it's important for this discussion, a diagram might help. -- MST
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