Xen devel - Nov 2006 - Information required on the pinning/unpinning of shadow.

Hi all,
Looking at the following code snippets could anybody explain what is the purpose
and
meaning of the pinning and unpinning and hooking/unhook of the shadow/subshadow?
Please correct my question if it seems some information missing in it.

xen-3.0.3_0-src\xen\arch\x86\mm\shadow\common.c
void shadow_prealloc(struct domain *d, unsigned int order)
{
    /* Need a vpcu for calling unpins; for now, since we don''t have
     * per-vcpu shadows, any will do */
    struct vcpu *v = d->vcpu[0];
   ....
            /* For PAE, we need to unpin each subshadow on this shadow */
            SHADOW_INTERNAL_NAME(sh_unpin_all_l3_subshadows,3,3)(v, smfn);
        } 
        else 
#endif /* 32-bit code always takes this branch */
        {
            /* Unpin this top-level shadow */
            sh_unpin(v, smfn);
       .....
         shadow_unhook_mappings(v, smfn);
....

Also in the following code what is PGC_SH_pinned doing internally?
xen-3.0.3_0-src\xen\arch\x86\mm\shadow\private.h
/* Unpin a shadow page: unset the pin bit and release the extra ref. */
static inline void sh_unpin(struct vcpu *v, mfn_t smfn)
{
    struct page_info *page;
    ASSERT(mfn_valid(smfn));
    page = mfn_to_page(smfn);
    if ( page->count_info & PGC_SH_pinned )
    {
        page->count_info &= ~PGC_SH_pinned;
        sh_put_ref(v, smfn, 0);
    }
}
xen-3.0.3_0-src\xen\include\asm-x86\mm.h
#define PGC_SH_pinned         (1U<<27)

With Best Regards,
Aditya Shevalkar.




		
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