Re: [Qemu-devel] rfc: vhost user enhancements for vm2vm communication

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Sep 03, 2015 at 10:21:28AM +0200, Jan Kiszka wrote:
> On 2015-09-03 10:08, Michael S. Tsirkin wrote:
> > On Tue, Sep 01, 2015 at 06:28:28PM +0200, Jan Kiszka wrote:
> >> On 2015-09-01 18:02, Michael S. Tsirkin wrote:
> >>> On Tue, Sep 01, 2015 at 05:34:37PM +0200, Jan Kiszka wrote:
> >>>> On 2015-09-01 16:34, Michael S. Tsirkin wrote:
> >>>>> On Tue, Sep 01, 2015 at 04:09:44PM +0200, Jan Kiszka wrote:
> >>>>>> On 2015-09-01 11:24, Michael S. Tsirkin wrote:
> >>>>>>> On Tue, Sep 01, 2015 at 11:11:52AM +0200, Jan Kiszka wrote:
> >>>>>>>> On 2015-09-01 10:01, Michael S. Tsirkin wrote:
> >>>>>>>>> On Tue, Sep 01, 2015 at 09:35:21AM +0200, Jan Kiszka wrote:
> >>>>>>>>>> Leaving all the implementation and interface details aside, this
> >>>>>>>>>> discussion is first of all about two fundamentally different approaches:
> >>>>>>>>>> static shared memory windows vs. dynamically remapped shared windows (a
> >>>>>>>>>> third one would be copying in the hypervisor, but I suppose we all agree
> >>>>>>>>>> that the whole exercise is about avoiding that). Which way do we want or
> >>>>>>>>>> have to go?
> >>>>>>>>>>
> >>>>>>>>>> Jan
> >>>>>>>>>
> >>>>>>>>> Dynamic is a superset of static: you can always make it static if you
> >>>>>>>>> wish. Static has the advantage of simplicity, but that's lost once you
> >>>>>>>>> realize you need to invent interfaces to make it work.  Since we can use
> >>>>>>>>> existing IOMMU interfaces for the dynamic one, what's the disadvantage?
> >>>>>>>>
> >>>>>>>> Complexity. Having to emulate even more of an IOMMU in the hypervisor
> >>>>>>>> (we already have to do a bit for VT-d IR in Jailhouse) and doing this
> >>>>>>>> per platform (AMD IOMMU, ARM SMMU, ...) is out of scope for us. In that
> >>>>>>>> sense, generic grant tables would be more appealing.
> >>>>>>>
> >>>>>>> That's not how we do things for KVM, PV features need to be
> >>>>>>> modular and interchangeable with emulation.
> >>>>>>
> >>>>>> I know, and we may have to make some compromise for Jailhouse if that
> >>>>>> brings us valuable standardization and broad guest support. But we will
> >>>>>> surely not support an arbitrary amount of IOMMU models for that reason.
> >>>>>>
> >>>>>>>
> >>>>>>> If you just want something that's cross-platform and easy to
> >>>>>>> implement, just build a PV IOMMU. Maybe use virtio for this.
> >>>>>>
> >>>>>> That is likely required to keep the complexity manageable and to allow
> >>>>>> static preconfiguration.
> >>>>>
> >>>>> Real IOMMU allow static configuration just fine. This is exactly
> >>>>> what VFIO uses.
> >>>>
> >>>> Please specify more precisely which feature in which IOMMU you are
> >>>> referring to. Also, given that you refer to VFIO, I suspect we have
> >>>> different thing in mind. I'm talking about an IOMMU device model, like
> >>>> the one we have in QEMU now for VT-d. That one is not at all
> >>>> preconfigured by the host for VFIO.
> >>>
> >>> I really just mean that VFIO creates a mostly static IOMMU configuration.
> >>>
> >>> It's configured by the guest, not the host.
> >>
> >> OK, that resolves my confusion.
> >>
> >>>
> >>> I don't see host control over configuration as being particularly important.
> >>
> >> We do, see below.
> >>
> >>>
> >>>
> >>>>>
> >>>>>> Well, we could declare our virtio-shmem device to be an IOMMU device
> >>>>>> that controls access of a remote VM to RAM of the one that owns the
> >>>>>> device. In the static case, this access may at most be enabled/disabled
> >>>>>> but not moved around. The static regions would have to be discoverable
> >>>>>> for the VM (register read-back), and the guest's firmware will likely
> >>>>>> have to declare those ranges reserved to the guest OS.
> >>>>>> In the dynamic case, the guest would be able to create an alternative
> >>>>>> mapping.
> >>>>>
> >>>>>
> >>>>> I don't think we want a special device just to support the
> >>>>> static case. It might be a bit less code to write, but
> >>>>> eventually it should be up to the guest.
> >>>>> Fundamentally, it's policy that host has no business
> >>>>> dictating.
> >>>>
> >>>> "A bit less" is to be validated, and I doubt its just "a bit". But if
> >>>> KVM and its guests will also support some PV-IOMMU that we can reuse for
> >>>> our scenarios, than that is fine. KVM would not have to mandate support
> >>>> for it while we would, that's all.
> >>>
> >>> Someone will have to do this work.
> >>>
> >>>>>
> >>>>>> We would probably have to define a generic page table structure
> >>>>>> for that. Or do you rather have some MPU-like control structure in mind,
> >>>>>> more similar to the memory region descriptions vhost is already using?
> >>>>>
> >>>>> I don't care much. Page tables use less memory if a lot of memory needs
> >>>>> to be covered. OTOH if you want to use virtio (e.g. to allow command
> >>>>> batching) that likely means commands to manipulate the IOMMU, and
> >>>>> maintaining it all on the host. You decide.
> >>>>
> >>>> I don't care very much about the dynamic case as we won't support it
> >>>> anyway. However, if the configuration concept used for it is applicable
> >>>> to static mode as well, then we could reuse it. But preconfiguration
> >>>> will required register-based region description, I suspect.
> >>>
> >>> I don't know what you mean by preconfiguration exactly.
> >>>
> >>> Do you want the host to configure the IOMMU? Why not let the
> >>> guest do this?
> >>
> >> We simply freeze GPA-to-HPA mappings during runtime. Avoids having to
> >> validate and synchronize guest-triggered changes.
> > 
> > Fine, but this assumes guest does very specific things, right?
> > E.g. should guest reconfigure device's BAR, you would have
> > to change GPA to HPA mappings?
> > 
> 
> Yes, that's why we only support size exploration, not reallocation.
> 
> > 
> >>>>>
> >>>>>> Also not yet clear to me are how the vhost-pci device and the
> >>>>>> translations it will have to do should look like for VM2.
> >>>>>
> >>>>> I think we can use vhost-pci BAR + VM1 bus address as the
> >>>>> VM2 physical address. In other words, all memory exposed to
> >>>>> virtio-pci by VM1 through it's IOMMU is mapped into BAR of
> >>>>> vhost-pci.
> >>>>>
> >>>>> Bus addresses can be validated to make sure they fit
> >>>>> in the BAR.
> >>>>
> >>>> Sounds simple but may become challenging for VMs that have many of such
> >>>> devices (in order to connect to many possibly large VMs).
> >>>
> >>> You don't need to be able to map all guest memory if you know
> >>> guest won't try to allow device access to all of it.
> >>> It's a question of how good is the bus address allocator.
> >>
> >> But those BARs need to allocate a guest-physical address range as large
> >> as the other guest's RAM is, possibly even larger if that RAM is not
> >> contiguous, and you can't put other resources into potential holes
> >> because VM2 does not know where those holes will be.
> > 
> > No - only the RAM that you want addressable by VM2.
> 
> That's in the hand of VM1, not VM2 or the hypervisor, in case of
> reconfigurable mapping. It's indeed a non-issue in our static case.
> 
> > 
> > IOW if you wish, you actually can create a shared memory device,
> > make it accessible to the IOMMU and place some or all
> > data there.
> > 
> 
> Actually, that could also be something more sophisticated, including
> virtio-net, IF that device will be able to express its DMA window
> restrictions (a bit like 32-bit PCI devices being restricted to <4G
> addresses or ISA devices <1M).
> 
> Jan

Actually, it's the bus restriction, not the device restriction.

So if you want to use bounce buffers in the name of security or
real-time requirements, you should be able to do this if virtio uses the
DMA API.


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