Re: [PATCH 3/7] xen/p2m: put reference for superpage

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, May 09, 2024 at 10:50:56AM +0100, Julien Grall wrote:
> 
> 
> On 09/05/2024 09:13, Roger Pau Monné wrote:
> > On Wed, May 08, 2024 at 11:11:04PM +0100, Julien Grall wrote:
> > Also the interactions with the remote domain would need to be audited,
> > as the remote domain shattering the superpage would need to be
> > replicated in the mapping side in order to account for the changes.
> 
> ... I don't understand this one. How is this different from today's where a
> domain can foreign map a 2MB which may be using a superpage in the remote
> domain?

Hm, right, I was wrong with that I think, as long as proper references
as taken for the superpage entries it should be fine.

> >  Not sure all paths will be easy to
> > audit for preemption if it's more than relinquish_p2m_mapping() that
> > you need to adjust.
> 
> I thought about it yesterday. But I came to the conclusion that if we have
> any concern about removing 1GB foreign superpage then we would already have
> the problem today as a domain can map contiguously 1GB worth of foreign
> mapping using small pages.

Yeah, but in that case addition or removal is done in 4K chunks, and
hence we can preempt during the operation.

OTOH for 1GB given the code here the page could be freed in one go,
without a chance of preempting the operation.

Maybe you have to shatter superpages into 4K entries and then remove
them individually, as to allow for preemption to be possible by
calling put_page() for each 4K chunk?

Thanks, Roger.