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Re: [Stratos-dev] Xen Rust VirtIO demos work breakdown for Project Stratos

On 05.10.21 00:53, Stefano Stabellini wrote:

Hi Stefano, all

On Sat, 2 Oct 2021, Oleksandr Tyshchenko wrote:
On Sat, Oct 2, 2021 at 2:58 AM Stefano Stabellini <sstabellini@xxxxxxxxxx> 

Hi Stefano, all

[Sorry for the possible format issues]
[I have CCed Julien]

       On Tue, 28 Sep 2021, Oleksandr Tyshchenko wrote:
       > On Tue, Sep 28, 2021 at 9:26 AM Stefano Stabellini 
<sstabellini@xxxxxxxxxx> wrote:
       > Hi Stefano, all
       > [Sorry for the possible format issues]
       >       On Mon, 27 Sep 2021, Christopher Clark wrote:
       >       > On Mon, Sep 27, 2021 at 3:06 AM Alex Bennée via Stratos-dev 
<stratos-dev@xxxxxxxxxxxxxxxxxxx> wrote:
       >       >
       >       >       Marek Marczykowski-Górecki 
<marmarek@xxxxxxxxxxxxxxxxxxxxxx> writes:
       >       >
       >       >       > [[PGP Signed Part:Undecided]]
       >       >       > On Fri, Sep 24, 2021 at 05:02:46PM +0100, Alex Bennée 
       >       >       >> Hi,
       >       >       >
       >       >       > Hi,
       >       >       >
       >       >       >> 2.1 Stable ABI for foreignmemory mapping to non-dom0 
       >       >       >> 
       >       >       >>
       >       >       >>   Currently the foreign memory mapping support only 
works for dom0 due
       >       >       >>   to reference counting issues. If we are to support 
backends running in
       >       >       >>   their own domains this will need to get fixed.
       >       >       >>
       >       >       >>   Estimate: 8w
       >       >       >>
       >       >       >>
       >       >       >> [STR-57] <https://linaro.atlassian.net/browse/STR-57>
       >       >       >
       >       >       > I'm pretty sure it was discussed before, but I can't 
find relevant
       >       >       > (part of) thread right now: does your model assumes 
the backend (running
       >       >       > outside of dom0) will gain ability to map (or access 
in other way)
       >       >       > _arbitrary_ memory page of a frontend domain? Or 
worse: any domain?
       >       >
       >       >       The aim is for some DomU's to host backends for other 
DomU's instead of
       >       >       all backends being in Dom0. Those backend DomU's would 
have to be
       >       >       considered trusted because as you say the default memory 
model of VirtIO
       >       >       is to have full access to the frontend domains memory 
       >       >
       >       >
       >       > I share Marek's concern. I believe that there are Xen-based 
systems that will want to run guests using VirtIO devices
       >       extending
       >       > this level of trust to the backend domains.
       >       >From a safety perspective, it would be challenging to deploy a 
       >       with privileged backends. From a safety perspective, it would be 
a lot
       >       easier if the backend were unprivileged.
       >       This is one of those times where safety and security 
requirements are
       >       actually aligned.
       > Well, the foreign memory mapping has one advantage in the context of 
Virtio use-case
       > which is that Virtio infrastructure in Guest doesn't require any 
modifications to run on top Xen.
       > The only issue with foreign memory here is that Guest memory actually 
mapped without its agreement
       > which doesn't perfectly fit into the security model. (although there 
is one more issue with XSA-300,
       > but I think it will go away sooner or later, at least there are some 
attempts to eliminate it).
       > While the ability to map any part of Guest memory is not an issue for 
the backend running in Dom0
       > (which we usually trust), this will certainly violate Xen security 
model if we want to run it in other
       > domain, so I completely agree with the existing concern.

       Yep, that's what I was referring to.

       > It was discussed before [1], but I couldn't find any decisions 
regarding that. As I understand,
       > the one of the possible ideas is to have some entity in Xen (PV 
       > that works in protection mode, so it denies all foreign mapping 
requests from the backend running in DomU
       > by default and only allows requests with mapping which were 
*implicitly* granted by the Guest before.
       > For example, Xen could be informed which MMIOs hold the queue PFN and 
notify registers
       > (as it traps the accesses to these registers anyway) and could 
theoretically parse the frontend request
       > and retrieve descriptors to make a decision which GFNs are actually 
       > I can't say for sure (sorry not familiar enough with the topic), but 
implementing the virtio-iommu device
       > in Xen we could probably avoid Guest modifications at all. Of course, 
for this to work
       > the Virtio infrastructure in Guest should use DMA API as mentioned in 
       > Would the “restricted foreign mapping” solution retain the Xen 
security model and be accepted
       > by the Xen community? I wonder, has someone already looked in this 
direction, are there any
       > pitfalls here or is this even feasible?
       > [1] 

       The discussion that went further is actually one based on the idea that
       there is a pre-shared memory area and the frontend always passes
       addresses from it. For ease of implementation, the pre-shared area is
       the virtqueue itself so this approach has been called "fat virtqueue".
       But it requires guest modifications and it probably results in
       additional memory copies.

I got it. Although we would need to map that pre-shared area anyway (I presume it could be done at once during initialization), I think it
much better than
map arbitrary pages at runtime.
Yeah that's the idea

If there is a way for Xen to know the pre-shared area location in advance it 
will be able to allow mapping
this region only and deny other attempts.
No, but there are patches (not yet upstream) to introduce a way to
pre-share memory regions between VMs using xl:

So I think it would probably be the other way around: xen/libxl
advertises on device tree (or ACPI) the presence of the pre-shared
regions to both domains. Then frontend and backend would start using it.

Thank you for the explanation. I remember this series has already appeared in ML. If I got the idea correctly this way we won't need to map the foreign memory from the backend at all (I assume this eliminates security concern?). It looks like the every pre-shared region (described in config file) is mapped by the toolstack at the domains creation time and the details of this region are also written to the Xenstore. All what backend needs to do is to map the region into its address space (via mmap). For this to work the guest should allocate virtqueue from Xen specific reserved memory [1].

[1] https://www.kernel.org/doc/Documentation/devicetree/bindings/reserved-memory/xen%2Cshared-memory.txt

       I am not sure if the approach you mentioned could be implemented
       completely without frontend changes. It looks like Xen would have to
       learn how to inspect virtqueues in order to verify implicit grants
       without frontend changes.

I looked through the virtio-iommu specification and corresponding Linux driver but I am sure I don't see all the challenges and pitfalls.
Having a limited knowledge of IOMMU infrastructure in Linux, below is just my 
guess, which might be wrong.

1. I think, if we want to avoid frontend changes the backend in Xen would need 
to fully conform to the specification, I am afraid that
besides just inspecting virtqueues, the backend needs to properly and 
completely emulate the virtio device, handle shadow page tables, etc.
Otherwise we might break the guest. I expect a huge amount of work to implement 
this properly.
Yeah, I think we would want to stay away from shadow pagetables unless
we are really forced to go there.

2. Also, if I got the things correctly, it looks like when enabling virtio-iommu, 
all addresses passed in requests to the virtio devices
behind the virtio-iommu will be in guest virtual address space (IOVA). So we 
would need to find a way for userspace (if the backend is
IOREQ server) to translate them to guest physical addresses (IPA) via these 
shadow page tables in the backend in front of mapping them via
foreign memory map calls. So I expect Xen, toolstack and Linux privcmd driver 
changes and additional complexity taking into account how the
data structures could be accessed (data structures being continuously in IOVA, 
could be discontinuous in IPA, indirect table descriptors,
I am wondering, would it be possible to have identity IOMMU mapping (IOVA == 
GPA) at the guest side but without bypassing an IOMMU, as we
need the virtio-iommu frontend to send map/unmap requests, can we control this 
behaviour somehow?
I think this would simplify things.
None of the above looks easy. I think you are right that we would need
IOVA == GPA to make the implementation feasible and with decent

Yes. Otherwise, I am afraid, the implementation is going to be quite difficult with questionable performance at the end.

I found out that IOMMU domain in Linux can be identity mapped (IOMMU_DOMAIN_IDENTITY - DMA addresses are system physical addresses) and this can be controlled via cmd line. I admit I didn't test, but from the IOMMU framework code it looks like that driver's map/unmap callback won't be called in this mode and as the result the IOMMU mapping never reaches the backend. Unfortunately, this is not what we want as we won't have any understating what the GFNs are...

But if we need a spec change, then I think Juergen's
proposal of introducing a new transport that uses grant table references
instead of GPAs is worth considering.

Agree, if we the spec changes cannot be avoided then yes.

3. Also, we would probably want to have a single virtio-iommu device instance 
per guest, so all virtio devices which belong to this guest
will share the IOMMU mapping for the optimization purposes. For this to work 
all virtio devices inside a guest should be attached to the
same IOMMU domain. Probably, we could control that, but I am not 100% sure.


Oleksandr Tyshchenko



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