Re: [Xen-devel] [PATCH RESEND v5 4/6] xen/arm: Implement virtual-linear page table for guest p2m mapping in live migration

[Jaeyong Yoo <jaeyong.yoo@xxxxxxxxxxx>]

> > > +
> > > +    second_lvl_page = alloc_domheap_pages(NULL, 1, 0);
> >
> > There'd be no harm in allocating the two pages separately I think, and
> > avoiding the possiblilty of an order 1 allocation failing.
> >
> > But if you do allocate them contiguously then d->arch.dirty.second_lvl
> > can be a simple lpae_t * and you can just access offsets 0..1023
> > without worrying about managing the array of two pointers.
> >
> > As it stands I think you have the worst of both worlds.
> >
> > We could also just consider allocating all firstlevel p2m pages from
> > the xenheap instead of the domheap.
> 
> OK. xenheap allocation sounds good. I was not sure if I can use xenheap
> for the one that looks like 'domain-specific' purpose. If it's OK, it
> would be much better code readability.
> 
> >
> > > +    if ( second_lvl_page == NULL )
> > > +        return -ENOMEM;
> > > +
> > > +    /* First level p2m is 2 consecutive pages */
> > > +    d->arch.dirty.second_lvl[0] = map_domain_page_global(
> > > +
page_to_mfn(second_lvl_page) );
> > > +    d->arch.dirty.second_lvl[1] = map_domain_page_global(
> > > +
> > > + page_to_mfn(second_lvl_page+1) );
> > > +
> > > +    first[0] = __map_domain_page(p2m->first_level);
> > > +    first[1] = __map_domain_page(p2m->first_level+1);
> > > +    for ( i = gp2m_start_index; i < gp2m_end_index; ++i )
> >
> > Can't this just be a loop over 0..1023 and avoid all this modular
> > arithmetic:
> 
> Sure.

CTTOI, since p2m->first_level consists of 2 pages, we can't just loop over
0..1023,
unless we have an improved map_domain_page that can map consecutive pages. 
One way to avoid the modular arithmetic is to use nested loop; loop over
first_level
and then loop inside a first_level page. I think this one (nested loop) is
better than
modular arithmetic.

Jaeyong



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