Re: [Xen-devel] [PATCH v5 2/4] xen/arm: introduce a generic p2m walker and use it in p2m_lookup

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 10 Sep 2013, Ian Campbell wrote:
> This function is not actually all the p2m specific in the end, by using
> lpae_t.walk instead of lpae_t.pt (see dump_pt_walk) you could make it
> useful for e.g. hyp pagetable walks too. (e.g. dump_hyp_walk). In theory
> it could also be used for LPAE guest walks too, but we'd need a separate
> walker for non-LPAE guests.

I could move the function somewhere more generic but..

> If we wanted to replace dump_hyp_walk with this then calling the
> callback for each level would be required.

.. even though the difference is not likely to be big, still it would
slow down p2m_walker, that could even be called as often as twice per DMA
request.
Maybe it's best to keep it as it is?


> >  
> >      spin_lock(&p2m->lock);
> >  
> >      first = __map_domain_page(p2m->first_level);
> >  
> > -    pte = first[first_table_offset(paddr)];
> > -    if ( !pte.p2m.valid || !pte.p2m.table )
> > -        goto done;
> > +    if ( !first ||
> > +         !first[first_table_offset(paddr)].p2m.valid )
> > +        goto err;
> 
> Why is the first level handled specially outside the loop? What happens
> if order is such that we span multiple first level entries?

I think that the assumption in the original code was that the size
couldn't be larger than 1GB. It isn't the case anymore, I'll fix this.


> > -    /* This bit must be one in the level 3 entry */
> > -    if ( !pte.p2m.table )
> > -        pte.bits = 0;
> > +        if ( cur_first_offset != first_table_offset(paddr) )
> > +        {
> > +            if (second) unmap_domain_page(second);
> > +            second = 
> > map_domain_page(first[first_table_offset(paddr)].p2m.base);
> > +            cur_first_offset = first_table_offset(paddr);
> > +        }
> > +        level++;
> > +        if ( !second ||
> 
> ASSERT(second) seems reasonable here, I think, since it would indicate
> we had screwed up the p2m pretty badly.

I think it's possible: what if a memory range is simply missing from the
p2m? The second level pagetable pages could be missing too.

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