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Re: Virtio in Xen on Arm (based on IOREQ concept)

Julien Grall <julien@xxxxxxx> writes:

> Hi Stefano,
> On 20/07/2020 21:37, Stefano Stabellini wrote:
>> On Mon, 20 Jul 2020, Roger Pau Monné wrote:
>>> On Mon, Jul 20, 2020 at 10:40:40AM +0100, Julien Grall wrote:
>>>> On 20/07/2020 10:17, Roger Pau Monné wrote:
>>>>> On Fri, Jul 17, 2020 at 09:34:14PM +0300, Oleksandr wrote:
>>>>>> On 17.07.20 18:00, Roger Pau Monné wrote:
>>>>>>> On Fri, Jul 17, 2020 at 05:11:02PM +0300, Oleksandr Tyshchenko wrote:
>>>>>>> Do you have any plans to try to upstream a modification to the VirtIO
>>>>>>> spec so that grants (ie: abstract references to memory addresses) can
>>>>>>> be used on the VirtIO ring?
>>>>>> But VirtIO spec hasn't been modified as well as VirtIO infrastructure in 
>>>>>> the
>>>>>> guest. Nothing to upsteam)
>>>>> OK, so there's no intention to add grants (or a similar interface) to
>>>>> the spec?
>>>>> I understand that you want to support unmodified VirtIO frontends, but
>>>>> I also think that long term frontends could negotiate with backends on
>>>>> the usage of grants in the shared ring, like any other VirtIO feature
>>>>> negotiated between the frontend and the backend.
>>>>> This of course needs to be on the spec first before we can start
>>>>> implementing it, and hence my question whether a modification to the
>>>>> spec in order to add grants has been considered.
>>>> The problem is not really the specification but the adoption in the
>>>> ecosystem. A protocol based on grant-tables would mostly only be used by 
>>>> Xen
>>>> therefore:
>>>>     - It may be difficult to convince a proprietary OS vendor to invest
>>>> resource on implementing the protocol
>>>>     - It would be more difficult to move in/out of Xen ecosystem.
>>>> Both, may slow the adoption of Xen in some areas.
>>> Right, just to be clear my suggestion wasn't to force the usage of
>>> grants, but whether adding something along this lines was in the
>>> roadmap, see below.
>>>> If one is interested in security, then it would be better to work with the
>>>> other interested parties. I think it would be possible to use a virtual
>>>> IOMMU for this purpose.
>>> Yes, I've also heard rumors about using the (I assume VirtIO) IOMMU in
>>> order to protect what backends can map. This seems like a fine idea,
>>> and would allow us to gain the lost security without having to do the
>>> whole work ourselves.
>>> Do you know if there's anything published about this? I'm curious
>>> about how and where in the system the VirtIO IOMMU is/should be
>>> implemented.
>> Not yet (as far as I know), but we have just started some discussons on
>> this topic within Linaro.
>> You should also be aware that there is another proposal based on
>> pre-shared-memory and memcpys to solve the virtio security issue:
>> https://marc.info/?l=linux-kernel&m=158807398403549
>> It would be certainly slower than the "virtio IOMMU" solution but it
>> would take far less time to develop and could work as a short-term
>> stop-gap.
> I don't think I agree with this blank statement. In the case of "virtio 
> IOMMU", you would need to potentially map/unmap pages every request 
> which would result to a lot of back and forth to the hypervisor.

Can a virtio-iommu just set bounds when a device is initialised as to
where memory will be in the kernel address space?

> So it may turn out that pre-shared-memory may be faster on some setup.

Certainly having to update the page permissions every transaction is
going to be to slow for soemthing that wants to avoid the performance
penalty of a bounce buffer.

Alex Bennée



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