Re: Virtio on Xen with Rust

[Viresh Kumar <viresh.kumar@xxxxxxxxxx>]

On 23-06-22, 15:47, Oleksandr Tyshchenko wrote:
> Below is my understanding, which might be wrong.
> 
> I am not sure about x86, there are some moments with its modes, for example
> PV guests should always use grants for virtio, but on Arm (which guest type
> is HVM):
> 1. If you run backend(s) in dom0 which is trusted by default, you don't
> necessarily need to use grants for the virtio so you will be able to map
> what you need in advance using foreign mappings.
> 2. If you run backend(s) in another domain *which you trust* and you don't
> want to use grants for the virtio, I think, you also will be able to map in
> advance using foreign mappings, but for that you will need a security
> policy to allow your backend's domain to map arbitrary guest pages.
> 3. If you run backend(s) in non-trusted domain, you will have to use grants
> for the virtio, so there is no way to map in advance, only to map at the
> runtime what was previously granted by the guest and umap right after using
> it.
> 
> These is another method how to restrict backend without modifying guest
> which is CONFIG_DMA_RESTRICTED_POOL in Linux, but this includes memcpy in
> the guest and requires some support in toolstack to make it work, but I
> wouldn't
> suggest it as the usage of grants for the virtio is better (and already in
> upsteam).

Yeah, above looks okay.

> Regarding your previous attempt to map 512MB by using grants, what I
> understand from the error message is that Xen complains that the passed
> grant ref is bigger than the current number of grant table entries.
> Now I am wondering where do these 0x40000 - 0x5ffff grant refs (which
> backend tries to map in a single call) come from, are they really were
> previously granted by the guest and passed to the backend in a
> single request?

I just tried to map everything in one go, just like map in advance.
Yeah, the whole idea is faulty :)

The guest never agreed to it.

> If the answer is yes, then what does gnttab_usage_print_all() say (key 'g'
> in Xen console)? I expect there should be a lot of Xen messages like
> "common/grant_table.c:1882:d2v3 Expanding d2 grant table from 28 to 29
> frames. Do you see them?

I am not sure if there were other messages, but anyway this doesn't
bother me now as the whole thing was wrong to begin with. :)

-- 
viresh