Re: [Xen-devel] [RFC 15/22] xen/arm: p2m: Re-implement relinquish_p2m_mapping using p2m_get_entry

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 05/09/16 22:58, Stefano Stabellini wrote:
> On Thu, 28 Jul 2016, Julien Grall wrote:
> > The current implementation of relinquish_p2m_mapping is modifying the
> > page table to erase the entry one by one. However, this is not necessary
> > because the domain is not running anymore and therefore will speed up
> > the domain destruction.
> Could you please elaborate on this? Who is going to remove the p2m
> entries if not this function?
The current version of relinquish is removing the reference on the page 
and then invalidate the entry (which may involve a cache flush).
As the page tables are not used anymore by the hardware, the latter 
action is not necessary. This is an optimization because flushing the 
cache can be expensive. However as mentioned later in the commit 
message, we need to have a think on how the other helpers interact with 
the page table to avoid return wrong entry.
I am thinking to defer this optimization for the next release (i.e Xen 
4.9) to avoid rushing on it.
> > The function relinquish_p2m_mapping can be re-implemented using
> > p2m_get_entry by iterating over the range mapped and using the mapping
> > order given by the callee.
> >
> > Given that the preemption was chosen arbitrarily, it is no done on every
>                                                           ^ now?
Yes, will fix it in the next version.

Regards,

--
Julien Grall

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