[xen staging] xen/arm: mm: Encode existing constraints of the memory layout

[patchbot@xxxxxxx]

commit 1f410b0c7455748021be4ede59e7a0c0a2ffb1c4
Author:     Julien Grall <jgrall@xxxxxxxxxx>
AuthorDate: Thu Mar 3 20:14:53 2022 +0000
Commit:     Julien Grall <jgrall@xxxxxxxxxx>
CommitDate: Thu Mar 3 20:15:14 2022 +0000

    xen/arm: mm: Encode existing constraints of the memory layout
    
    The boot code expects the regions XEN_VIRT_START, FIXMAP_ADDR(0),
    BOOT_FDT_VIRT_START to use the same 0th (arm64 only) and 1st slot.
    
    Add some BUILD_BUG_ON() to confirm that. This is helpful if one wants
    to re-order the memory layout.
    
    Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
    Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
---
 xen/arch/arm/mm.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index 8a17222109..7b1f2f4906 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -211,6 +211,22 @@ static void __init __maybe_unused build_assertions(void)
 #ifdef CONFIG_DOMAIN_PAGE
     BUILD_BUG_ON(DOMHEAP_VIRT_START & ~FIRST_MASK);
 #endif
+    /*
+     * The boot code expects the regions XEN_VIRT_START, FIXMAP_ADDR(0),
+     * BOOT_FDT_VIRT_START to use the same 0th (arm64 only) and 1st
+     * slot in the page tables.
+     */
+#define CHECK_SAME_SLOT(level, virt1, virt2) \
+    BUILD_BUG_ON(level##_table_offset(virt1) != level##_table_offset(virt2))
+
+#ifdef CONFIG_ARM_64
+    CHECK_SAME_SLOT(zeroeth, XEN_VIRT_START, FIXMAP_ADDR(0));
+    CHECK_SAME_SLOT(zeroeth, XEN_VIRT_START, BOOT_FDT_VIRT_START);
+#endif
+    CHECK_SAME_SLOT(first, XEN_VIRT_START, FIXMAP_ADDR(0));
+    CHECK_SAME_SLOT(first, XEN_VIRT_START, BOOT_FDT_VIRT_START);
+
+#undef CHECK_SAME_SLOT
 }
 
 void dump_pt_walk(paddr_t ttbr, paddr_t addr,
--
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