Re: [PATCH 12/12] xen/arm: call iomem_permit_access for passthrough devices

[Julien Grall <julien@xxxxxxx>]

Hi,

On 30/04/2020 14:01, Julien Grall wrote:
> On 29/04/2020 21:47, Stefano Stabellini wrote:
> > On Wed, 15 Apr 2020, Julien Grall wrote: But doesn't it make sense to 
> > give domU permission if it is going to get
> > the memory mapped? But admittedly I can't think of something that would
> > break because of the lack of the iomem_permit_access call in this code
> > path.
> On Arm, the permissions are only useful if you plan you DomU to delegate 
> the regions to another domain. As your domain is not even aware it is 
> running on Xen (we don't expose 'xen' node in the DT), it makes little 
> sense to add the permission.
I actually found one use when helping a user last week. You can dump the 
list of MMIO regions assigned to a guest from Xen Console.
This will use d->iomem_caps that is modified via iomem_permit_access(). 
Without it, there is no easy way to confirm the list of MMIO regions 
assigned to a guest. Although...
> Even today, you can map IOMEM to a DomU and then revert the permission 
> right after. They IOMEM will still be mapped in the guest and it will 
> act normaly.
... this would not help the case where permissions are reverted. But I 
am assuming this shouldn't happen for Dom0less.
Stefano, I am not sure what's your plan for the series itself for Xen 
4.14. I think this patch could go in now. Any thoughts?
Cheers,

--
Julien Grall