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Subject: Re: Constraining where a guest may allocate virtio accessible resources

On 17.06.20 19:31, Alex BennÃe wrote:
> Hi,
> This follows on from the discussion in the last thread I raised:
>   Subject: Backend libraries for VirtIO device emulation
>   Date: Fri, 06 Mar 2020 18:33:57 +0000
>   Message-ID: <874kv15o4q.fsf@linaro.org>
> To support the concept of a VirtIO backend having limited visibility of
> a guests memory space there needs to be some mechanism to limit the
> where that guest may place things. A simple VirtIO device can be
> expressed purely in virt resources, for example:
>    * status, feature and config fields
>    * notification/doorbell
>    * one or more virtqueues
> Using a PCI backend the location of everything but the virtqueues it
> controlled by the mapping of the PCI device so something that is
> controllable by the host/hypervisor. However the guest is free to
> allocate the virtqueues anywhere in the virtual address space of system
> RAM.
> In theory this shouldn't matter because sharing virtual pages is just a
> matter of putting the appropriate translations in place. However there
> are multiple ways the host and guest may interact:
> QEMU sees a block of system memory in it's virtual address space that
> has a one to one mapping with the guests physical address space. If QEMU
> want to share a subset of that address space it can only realistically
> do it for a contiguous region of it's address space which implies the
> guest must use a contiguous region of it's physical address space.
> The situation here is broadly the same - although both QEMU and the
> guest are seeing a their own virtual views of a linear address space
> which may well actually be a fragmented set of physical pages on the
> host.
> KVM based guests have additional constraints if they ever want to access
> real hardware in the host as you need to ensure any address accessed by
> the guest can be eventually translated into an address that can
> physically access the bus which a device in one (for device
> pass-through). The area also has to be DMA coherent so updates from a
> bus are reliably visible to software accessing the same address space.
> * Xen (and other type-1's?)
> Here the situation is a little different because the guest explicitly
> makes it's pages visible to other domains by way of grant tables. The
> guest is still free to use whatever parts of its address space it wishes
> to. Other domains then request access to those pages via the hypervisor.
> In theory the requester is free to map the granted pages anywhere in
> its own address space. However there are differences between the
> architectures on how well this is supported.
> So I think this makes a case for having a mechanism by which the guest
> can restrict it's allocation to a specific area of the guest physical
> address space. The question is then what is the best way to inform the
> guest kernel of the limitation?
> Option 1 - Kernel Command Line
> ==============================
> This isn't without precedent - the kernel supports options like "memmap"
> which can with the appropriate amount of crafting be used to carve out
> sections of bad ram from the physical address space. Other formulations
> can be used to mark specific areas of the address space as particular
> types of memory.  
> However there are cons to this approach as it then becomes a job for
> whatever builds the VMM command lines to ensure the both the backend and
> the kernel know where things are. It is also very Linux centric and
> doesn't solve the problem for other guest OSes. Considering the rest of
> VirtIO can be made discover-able this seems like it would be a backward
> step.
> Option 2 - Additional Platform Data
> ===================================
> This would be extending using something like device tree or ACPI tables
> which could define regions of memory that would inform the low level
> memory allocation routines where they could allocate from. There is
> already of the concept of "dma-ranges" in device tree which can be a
> per-device property which defines the region of space that is DMA
> coherent for a device.
> There is the question of how you tie regions declared here with the
> eventual instantiating of the VirtIO devices?
> For a fully distributed set of backends (one backend per device per
> worker VM) you would need several different regions. Would each region
> be tied to each device or just a set of areas the guest would allocate
> from in sequence?
> Option 3 - Abusing PCI Regions
> ==============================
> One of the reasons to use the VirtIO PCI backend it to help with
> automatic probing and setup. Could we define a new PCI region which on
> backend just maps to RAM but from the front-ends point of view is a
> region it can allocate it's virtqueues? Could we go one step further and
> just let the host to define and allocate the virtqueue in the reserved
> PCI space and pass the base of it somehow?
> Options 4 - Extend VirtIO Config
> ================================
> Another approach would be to extend the VirtIO configuration and
> start-up handshake to supply these limitations to the guest. This could
> be handled by the addition of a feature bit (VIRTIO_F_HOST_QUEUE?) and
> additional configuration information.
> One problem I can foresee is device initialisation is usually done
> fairly late in the start-up of a kernel by which time any memory zoning
> restrictions will likely need to have informed the kernels low level
> memory management. Does that mean we would have to combine such a
> feature behaviour with a another method anyway?
> 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.

> Closing Thoughts and Open Questions
> ===================================
> Currently all of this is considering just virtqueues themselves but of
> course only a subset of devices interact purely by virtqueue messages.
> Network and Block devices often end up filling up additional structures
> in memory that are usually across the whole of system memory. To achieve
> better isolation you either need to ensure that specific bits of kernel
> allocation are done in certain regions (i.e. block cache in "shared"
> region) or implement some sort of bounce buffer [1] that allows you to bring
> data from backend to frontend (which is more like the channel concept of
> Xen's PV).

For [1], look at https://lkml.org/lkml/2020/3/26/700 or at
(which should be using swiotlb one day).

> I suspect the solution will end up being a combination of all of these
> approaches. There setup of different systems might mean we need a
> plethora of ways to carve out and define regions in ways a kernel can
> understand and make decisions about.
> I think there will always have to be an element of VirtIO config
> involved as that is *the* mechanism by which front/back end negotiate if
> they can get up and running in a way they are both happy with.
> One potential approach would be to introduce the concept of a region id
> at the VirtIO config level which is simply a reasonably unique magic
> number that virtio driver passes down into the kernel when requesting
> memory for it's virtqueues. It could then be left to the kernel to
> associate use that id when identifying the physical address range to
> allocate from. This seems a bit of a loose binding between the driver
> level and the kernel level but perhaps that is preferable to allow for
> flexibility about how such regions are discovered by kernels?
> I hope this message hasn't rambled on to much. I feel this is a complex
> topic and I'm want to be sure I've thought through all the potential
> options before starting to prototype a solution. For those that have
> made it this far the final questions are:
>   - is constraining guest allocation of virtqueues a reasonable requirement?
>   - could virtqueues ever be directly host/hypervisor assigned?
>   - should there be a tight or loose coupling between front-end driver
>     and kernel/hypervisor support for allocating memory?
> Of course if this is all solvable with existing code I'd be more than
> happy but please let me know how ;-)

Queues are a central element of virtio, but there is a (maintainability
& security) benefit if you can keep them away from the hosting
hypervisor, limit their interpretation and negotiation to the backend
driver in a host process or in a backend guest VM. So I would be careful
with coupling things too tightly.

One of the issues I see in virtio for use in minimalistic hypervisors is
the need to be aware of the different virtio devices when using PCI or
MMIO transports. That's where a shared memory transport come into play.


Siemens AG, Corporate Technology, CT RDA IOT SES-DE
Corporate Competence Center Embedded Linux

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