Re: [PATCH for-4.15 2/4] xen/iommu: x86: Free the IOMMU page-tables with the pgtables.lock held

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.12.2020 16:43, Julien Grall wrote:
> From: Julien Grall <jgrall@xxxxxxxxxx>
> 
> The pgtables.lock is protecting access to the page list pgtables.list.
> However, iommu_free_pgtables() will not held it. I guess it was assumed
> that page-tables cannot be allocated while the domain is dying.
> 
> Unfortunately, there is no guarantee that iommu_map() will not be
> called while a domain is dying (it looks like to be possible from
> XEN_DOMCTL_memory_mapping).

I'd rather disallow any new allocations for a dying domain, not
the least because ...

> So it would be possible to be concurrently
> allocate memory and free the page-tables.
> 
> Therefore, we need to held the lock when freeing the page tables.

... we should try to avoid holding locks across allocation /
freeing functions wherever possible.

As to where to place a respective check - I wonder if we wouldn't
be better off disallowing a majority of domctl-s (and perhaps
other operations) on dying domains. Thoughts?

Jan