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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Sun, 24 May 2020, Julien Grall wrote:
> 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.

Thank you for looking into this


> 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?

For the series: I have addresses all comments in my working tree except
for the ones on memory allocation (the thread "xen: introduce
reserve_heap_pages"). It looks like that part requires a complete
rewrite, and it seems that the new code is not trivial to write. So I am
thinking of not targeting 4.14. What do you think? Do you think the new
code should be "easy" enough that I could target 4.14?

For this patch: it is fine to go in now, doesn't have to wait for the
series.