Re: [Xen-devel] [PATCH 06/18] arm/altp2m: Add a(p2m) table flushing routines.

[Julien Grall <julien.grall@xxxxxxx>]

Hello Sergej,

On 04/07/16 13:12, Sergej Proskurin wrote:
> > +/* Reset this p2m table to be empty */
> > +static void p2m_flush_table(struct p2m_domain *p2m)
> > +{
> > +    struct page_info *top, *pg;
> > +    mfn_t mfn;
> > +    unsigned int i;
> > +
> > +    /* Check whether the p2m table has already been flushed before. */
> > +    if ( p2m->root == NULL)
> > +        return;
> > +
> > +    spin_lock(&p2m->lock);
> > +
> > +    /*
> > +     * "Host" p2m tables can have shared entries &c that need a bit more care
> > +     * when discarding them
> > +     */
> > +    ASSERT(!p2m_is_hostp2m(p2m));
> > +
> > +    /* Zap the top level of the trie */
> > +    top = p2m->root;
> > +
> > +    /* Clear all concatenated first level pages */
> > +    for ( i = 0; i < P2M_ROOT_PAGES; i++ )
> > +    {
> > +        mfn = _mfn(page_to_mfn(top + i));
> > +        clear_domain_page(mfn);
> > +    }
> > +
> > +    /* Free the rest of the trie pages back to the paging pool */
> > +    while ( (pg = page_list_remove_head(&p2m->pages)) )
> > +        if ( pg != top  )
> > +        {
> > +            /*
> > +             * Before freeing the individual pages, we clear them to prevent
> > +             * reusing old table entries in future p2m allocations.
> > +             */
> > +            mfn = _mfn(page_to_mfn(pg));
> > +            clear_domain_page(mfn);
> > +            free_domheap_page(pg);
> > +        }
> At this point, we prevent only the first root level page from being
> freed. In case there are multiple consecutive first level pages, one of
> them will be freed in the upper loop (and potentially crash the guest if
> the table is reused at a later point in time). However, testing for
> every concatenated page in the if clause of the while loop would further
> decrease the flushing performance. Thus, my question is, whether there
> is a good way to solve this issue?
The root pages are not part of p2m->pages, so there is no issue.

Regards,

--
Julien Grall

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