Re: [PATCH V2 21/23] xen/arm: Add mapcache invalidation handling

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.10.2020 10:41, Julien Grall wrote:
> On 16/10/2020 07:29, Jan Beulich wrote:
>> Given how p2m_free_entry() works (or is supposed to work in the
>> long run), is the new code you add guaranteed to only alter leaf
>> entries?
> 
> This path may also be called with tables. I think we want to move the 
> check in p2m_free_entry() so we can find the correct leaf type.
> 
>> If not, the freeing of page tables needs deferring until
>> after qemu has dropped its mappings.
> 
> Freeing the page tables doesn't release a page. So may I ask why we 
> would need to defer it?

Oh, sorry - qemu of course doesn't use the same p2m, so the
intermediate page tables are private to the subject guest.

>> And with there being refcounting only for foreign pages, how do
>> you prevent the freeing of the page just unmapped before qemu has
>> dropped its possible mapping?
> QEMU mappings can only be done using the foreign mapping interface. This 
> means that page reference count will be incremented for each QEMU 
> mappings. Therefore the page cannot disappear until QEMU dropped the 
> last reference.

Okay, sorry for the noise then.

Jan