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

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

Hi Julien,


On 07/04/2016 05:42 PM, Julien Grall wrote:
> 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.

Thanks for clearing that up. We have already discussed this in patch #04.

>
> Regards,
>

Thank you.

Best regards,
Sergej


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