[PATCH] x86/mm: Fix IS_ALIGNED() check in IS_LnE_ALIGNED()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The current CI failures happen to be a latent bug triggered by a narrow set of
properties of the initrd, which CI encountered by chance.

One step during boot involves constructing directmap mappings for modules.
With some probing at the point of creation, it is observed that there's a 4k
mapping missing towards the end of the initrd.

  (XEN) === Mapped Mod1 [0000000394001000, 00000003be1ff6dc] to Directmap
  (XEN) Probing paddr 394001000, va ffff830394001000
  (XEN) Probing paddr 3be1ff6db, va ffff8303be1ff6db
  (XEN) Probing paddr 3bdffffff, va ffff8303bdffffff
  (XEN) Probing paddr 3be001000, va ffff8303be001000
  (XEN) Probing paddr 3be000000, va ffff8303be000000
  (XEN) Early fatal page fault at e008:ffff82d04032014c (cr2=ffff8303be000000, 
ec=0000)

The conditions for this bug appear to be map_pages_to_xen() call with a non-2M
aligned start address, some number of full 2M pages, then a tail needing 4k
pages.

Anyway, the condition for spotting superpage boundaries in map_pages_to_xen()
is wrong.  The IS_ALIGNED() macro expects a power of two for the alignment
argument, and subtracts 1 itself.

Fixing this causes the failing case to now boot.

Fixes: 97fb6fcf26e8 ("x86/mm: introduce helpers to detect super page alignment")
Debugged-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Tested-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>

Judging by how IS_ALIGNED() is wrong, I think the pre-condition might be
exactly 4k past a 2M boundary, not just simply a non-2M boundary.
---
 xen/arch/x86/mm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 03b8319f7a9d..e0793ad3462b 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -5505,7 +5505,7 @@ int map_pages_to_xen(
                                                                 \
     ASSERT(!mfn_eq(m_, INVALID_MFN));                           \
     IS_ALIGNED(PFN_DOWN(v) | mfn_x(m_),                         \
-               (1UL << (PAGETABLE_ORDER * ((n) - 1))) - 1);     \
+               (1UL << (PAGETABLE_ORDER * ((n) - 1))));         \
 })
 #define IS_L2E_ALIGNED(v, m) IS_LnE_ALIGNED(v, m, 2)
 #define IS_L3E_ALIGNED(v, m) IS_LnE_ALIGNED(v, m, 3)

base-commit: 77be740e8182fa6b31291a8ae983d253187e9b50
-- 
2.39.5