[Xen-devel] page counts & type counts

["Mike Sun" <msun@xxxxxxxxxx>]

Hi,

I'm working with Xen 3.2 and having a hard time understanding what's
going on in this piece of code used to do ref and type counting.

int get_page_type(struct page_info *page, unsigned long type)
{
   ...
        else if ( unlikely((x & PGT_count_mask) == 0) )
        {
            struct domain *d = page_get_owner(page);

            /* Never allow a shadowed frame to go from type count 0 to 1 */
            if ( d && shadow_mode_enabled(d) )
                shadow_remove_all_shadows(d->vcpu[0], _mfn(page_to_mfn(page)));

Does this mean that every time a page goes from read-only to writable,
the shadow PTE mapping that page is destroyed and recreated?  In
particular, if I wanted to restore writable access to a page during
log-dirty mode (such as what occurs when a fault is taken and that
page is then marked dirty and RW is not revoked in _sh_propagate()),
will the shadow always be destroyed and then recreated?

>From what I understand, shadow_remove_all_shadows() will remove all
shadows of the gmfn, which should be a guest PT.  Basically, it'll
remove all shadows inserted in the shadow_hash table for that gmfn.
It IS NOT removing shadow PTEs mapping that gmfn.  Is this a correct
understanding?  I don't understand how get_page_type() would know in
the code that is there how the given page is actually a guest PT?

-- Mike

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