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

[Jan Kiszka]

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.

> 
>> 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.

> 
>> 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.

> 
>> 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).

> 
> 
> One issue to consider is that VM1 can trick VM2 into writing
> into bus address that isn't mapped in the IOMMU, or
> is mapped read-only.
> We probably would have to teach KVM to handle this somehow,
> e.g. exit to QEMU, or even just ignore. Maybe notify guest
> e.g. by setting a bit in the config space of the device,
> to avoid easy DOS.

Well, that would be trivial for VM1 to check if there are only one or
two memory windows. Relying on the hypervisor to handle it may be
unacceptable for real-time VMs.

Jan

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