[xen staging] x86/PV: assert page state in mark_pv_pt_pages_rdonly()

[patchbot@xxxxxxx]

commit 6b1ca51b1a91d002636518afe4a8a50ba7212495
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Aug 18 09:40:08 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Aug 18 09:40:08 2021 +0200

    x86/PV: assert page state in mark_pv_pt_pages_rdonly()
    
    About every time I look at dom0_construct_pv()'s "calculation" of
    nr_pt_pages I question (myself) whether the result is precise or merely
    an upper bound. I think it is meant to be precise, but I think we would
    be better off having some checking in place. Hence add ASSERT()s to
    verify that
    - all pages have a valid L1...Ln (currently L4) page table type and
    - no other bits are set, in particular the type refcount is still zero.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/pv/dom0_build.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/xen/arch/x86/pv/dom0_build.c b/xen/arch/x86/pv/dom0_build.c
index 778c863ea4..6145d4320b 100644
--- a/xen/arch/x86/pv/dom0_build.c
+++ b/xen/arch/x86/pv/dom0_build.c
@@ -59,6 +59,16 @@ static __init void mark_pv_pt_pages_rdonly(struct domain *d,
         l1e_remove_flags(*pl1e, _PAGE_RW);
         page = mfn_to_page(l1e_get_mfn(*pl1e));
 
+        /*
+         * Verify that
+         * - all pages have a valid L1...Ln page table type and
+         * - no other bits are set, in particular the type refcount is still
+         *   zero.
+         */
+        ASSERT((page->u.inuse.type_info & PGT_type_mask) >= PGT_l1_page_table);
+        ASSERT((page->u.inuse.type_info & PGT_type_mask) <= 
PGT_root_page_table);
+        ASSERT(!(page->u.inuse.type_info & ~PGT_type_mask));
+
         /* Read-only mapping + PGC_allocated + page-table page. */
         page->count_info         = PGC_allocated | 3;
         page->u.inuse.type_info |= PGT_validated | 1;
--
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