Hi,
At 18:09 -0500 on 04 Jan (1231092582), Mike Sun wrote:
> >> (XEN) sh error: sh_remove_shadows(): can't find all shadows of mfn
> >> 02673 (shadow_flags=00000008)
>
> Premature declaration of success. I'm still getting the panic above
> from this piece of code.
>
> SHADOW_FOREACH_L1E(sl1mfn, sl1e, 0, done_l1,
> {
> flags_l1 = shadow_l1e_get_flags(*sl1e);
> if ((flags_l1 & _PAGE_PRESENT) && !(flags_l1 & _PAGE_RW))
> {
> mfn = shadow_l1e_get_mfn(*sl1e);
> pfn = mfn_to_gfn(v->domain, mfn);
> log_dirty_lock(v->domain);
> if (mfn_valid(mfn) && VALID_M2P(pfn) &&
> sh_mfn_is_dirty(v->domain, mfn))
> {
> /* hack: because of the external mapping condition
> * ref count not incremented when this is called from
> * hypercall originating from dom0, need to do it
> * manually
> */
> struct page_info *page = mfn_to_page(mfn);
> if (((page->u.inuse.type_info & PGT_type_mask)
> == PGT_writable_page)
> && ((page->u.inuse.type_info & PGT_count_mask) ==
> 0)
That looks quite strange to me; it will select every dirty page that is
not currently writeable
- regardless of whether the guest asked for a writeable mapping
(e.g. user-mode read-only maps of kernel structures); and
- regardless of whether it's currently shadowed!
If you make a shadowed page writeable this way then the backstop check
in get_page_type() (which you asked about in another email) will force
it to be unshadowed immediately. If you do that to the currently in-use
top-level pagetable, you'll get the crash above.
I think you need to check for !sh_mfn_is_a_page_table(mfn) before trying
to make this writeable.
> && get_page_type(page, PGT_writable_page))
> {
> shadow_l1e_t rw_sl1e =
> shadow_l1e_add_flags(*sl1e, _PAGE_RW);
Did you by any chance make this shadow_l1e_add_flags() operator by
copying l1e_add_flags from asm/page.h? That's a macro which modifies
its arguments, which you are definitely not allowed to do to a live
shadow pagetable. :)
Cheers,
Tim.
> shadow_write_entries(sl1e, &rw_sl1e, 1, sl1mfn);
> cow.made_rw_count++;
> }
> }
> log_dirty_unlock(v->domain);
> }
> });
>
> Am I failing to update the page count somewhere? Or are there more
> conditions I need to check for the page that I'm adding a RW mapping
> to?
>
> Thanks,
> Mike
--
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]
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