Re: [PATCH] libxl: arm: Allow grant mappings for backends running on Dom0

[Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>]

Hello Viresh

[sorry for the possible format issues]

On Fri, May 5, 2023 at 9:19 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:

On 05-04-23, 05:12, Viresh Kumar wrote:
> On 04-04-23, 21:16, Oleksandr Tyshchenko wrote:
> > ok, probably makes sense
>
> While testing both foreign and grant mappings I stumbled upon another
> related problem. How do I control the creation of iommu node from
> guest configuration file, irrespective of the domain backend is
> running at ? This is what we have right now:
>
> - always create iommu nodes if backend-dom != 0
> - always create iommu nodes if forced_grant == 1
>
> what I need to cover is
> - don't create iommu nodes irrespective of the domain
>
> This is required if you want to test both foreign and grant memory
> allocations, with different guests kernels. i.e. one guest kernel for
> device with grant mappings and another guest for device with foreign
> mappings. There is no way, that I know of, to disable the creation of
> iommu nodes. Of course we would want to use the same images for kernel
> and other stuff, so this needs to be controlled from guest
> configuration file.

Any input on this please ?

I was going to propose an idea, but I have just realized that you already voiced it here [1] ))
So what you proposed there sounds reasonable to me.

I will just rephrase it according to my understanding:

We probably need to consider transforming your "forced_grant" to something three-state, for example
"grant_usage" (or "use_grant" as you suggested) which could be "default behaviour" or "always disabled", or "always enabled".

With "grant_usage=default" we will get exact what we have at the moment (only create iommu nodes if backend-domid != 0)
With "grant_usage=disabled" we will force grants to be always disabled (don't create iommu nodes irrespective of the domain)
With "grant_usage=enabled" we will force grants to be always enabled (always create iommu nodes irrespective of the domain)

[1] https://lore.kernel.org/xen-devel/20230505093835.jcbwo6zjk5hcjvsm@vireshk-i7/

 

--
viresh

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Regards,

Oleksandr Tyshchenko