[Xen-devel] Making pages writable again in paging_log_dirty mode

["Mike Sun" <msun@xxxxxxxxxx>]

Hi,

I'm working off the 3.2.x codebase.

I've seen how "sh_page_fault()" uses "_sh_propagate()" to restore
writable access to pages after they have been dirtied.  I have a
situation in which I'd like to restore writable access to a set of
pages that haven't necessarily been dirtied yet, or have been dirtied,
but have not caused a page fault which would then make them writable
again.  I'm trying to use a hypercall invoked from dom0 that would
provide a list of pages to make writable again in log_dirty_mode.

Looking at the code which removes all writable mappings to a page from
an L2E, I tried to do something like this, where I walk the shadow
page tables for the log_dirty_domain and restore writable access to
certain pages.  Unfortunately, I get errors about page_type_count
overflows and other kinds of type errors.

    SHADOW_FOREACH_L2E(sl2mfn, sl2e, 0, done_l2, v->domain,
    {
        flags_l2 = shadow_l2e_get_flags(*sl2e);
        if ((flags_l2 & _PAGE_PRESENT) && (flags_l2 & _PAGE_USER))
        {
            sl1mfn = shadow_l2e_get_mfn(*sl2e);
            SHADOW_FOREACH_L1E(sl1mfn, sl1e, 0, done_l1,
            {
                flags_l1 = shadow_l1e_get_flags(*sl1e);
                if ((flags_l1 & _PAGE_PRESENT) &&
                    (flags_l1 & _PAGE_USER) &&
                    !(flags_l1 & _PAGE_RW))
                {
                    mfn = shadow_l1e_get_mfn(*sl1e);
                    if (mfn_valid(mfn))
                    {
                        struct page_info *page = mfn_to_page(mfn);
                        if ((page->u.inuse.type_info & PGT_type_mask)
                            == PGT_writable_page)
                        {
                            pfn = mfn_to_gfn(v->domain, mfn);
                            if (VALID_M2P(pfn) && my_check(pfn))
                            {
                                shadow_l1e_t rw_sl1e =
                                    shadow_l1e_add_flags(*sl1e, _PAGE_RW);
                                (void)
                                    shadow_set_l1e(v, sl1e, rw_sl1e, sl1mfn);
                            }
                        }
                    }
                }
            });
        }
    });

I dug a bit more to make sure I was increment/decrementing the ref
counts correctly and noticed this in the code of "get_page_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)));

I'm confused about this piece of code?  Do we have to destroy all
shadows mapping that mfn if the page type changes?

Thanks,
Mike

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