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 06/09/2016 19:21, Stefano Stabellini wrote:
> On Tue, 6 Sep 2016, Julien Grall wrote:
> > 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.
> The idea is that nobody will remove the p2m entries, until the whole p2m
> is torn down (p2m_teardown)?
That the idea but I hadn't had time to confirm it was the case.

> > I am thinking to defer this optimization for the next release (i.e Xen 4.9) to
> > avoid rushing on it.
> If we are sure that there are no interactions with the p2m between the
> domain_relinquish_resources and the p2m_teardown call, then this is
> acceptable. Otherwise delaying this optimization is wiser.
I will delay the optimization.

Regards,


--
Julien Grall

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