[Xen-devel] [PATCH 14/17] xen/arm64: head: Remove ID map as soon as it is not used

[Julien Grall <julien.grall@xxxxxxx>]

The ID map may clash with other parts of the Xen virtual memory layout.
At the moment, Xen is handling the clash by only creating a mapping to
the runtime virtual address before enabling the MMU.

The rest of the mappings (such as the fixmap) will be mapped after the
MMU is enabled. However, the code doing the mapping is not safe as it
replace mapping without using the Break-Before-Make sequence.

As the ID map can be anywhere in the memory, it is easier to remove all
the entries added as soon as the ID map is not used rather than adding
the Break-Before-Make sequence everywhere.

It is difficult to track where exactly the ID map was created without a
full rework of create_page_tables(). Instead, introduce a new function
remove_id_map() will look where is the top-level entry for the ID map
and remove it.

The new function is only called for the boot CPU. Secondary CPUs will
switch directly to the runtime page-tables so there are no need to
remove the ID mapping. Note that this still doesn't make the Secondary
CPUs path safe but it is not making it worst.

---
    Note that the comment refers to the patch  "xen/arm: tlbflush: Rework
    TLB helpers" under review (see [1]).

    Furthermore, it is very likely we will need to re-introduce the ID
    map to cater secondary CPUs boot and suspend/resume. For now, the
    attempt is to make boot CPU path fully Arm Arm compliant.

[1] https://lists.xenproject.org/archives/html/xen-devel/2019-05/msg01134.html
---
 xen/arch/arm/arm64/head.S | 86 ++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 71 insertions(+), 15 deletions(-)

diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index 192af3e8a2..96e85f8834 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -300,6 +300,13 @@ real_start_efi:
         ldr   x0, =primary_switched
         br    x0
 primary_switched:
+        /*
+         * The ID map may clash with other parts of the Xen virtual memory
+         * layout. As it is not used anymore, remove it completely to
+         * avoid having to worry about replacing existing mapping
+         * afterwards.
+         */
+        bl    remove_id_map
         bl    setup_fixmap
 #ifdef CONFIG_EARLY_PRINTK
         /* Use a virtual address to access the UART. */
@@ -632,10 +639,68 @@ enable_mmu:
         ret
 ENDPROC(enable_mmu)
 
+/*
+ * Remove the ID map for the page-tables. It is not easy to keep track
+ * where the ID map was mapped, so we will look for the top-level entry
+ * exclusive to the ID Map and remove it.
+ *
+ * Inputs:
+ *   x19: paddr(start)
+ *
+ * Clobbers x0 - x1
+ */
+remove_id_map:
+        /*
+         * Find the zeroeth slot used. Remove the entry from zeroeth
+         * table if the slot is not 0. For slot 0, the ID map was either
+         * done in first or second table.
+         */
+        lsr   x1, x19, #ZEROETH_SHIFT   /* x1 := zeroeth slot */
+        cbz   x1, 1f
+        /* It is not in slot 0, remove the entry */
+        ldr   x0, =boot_pgtable         /* x0 := root table */
+        str   xzr, [x0, x1, lsl #3]
+        b     id_map_removed
+
+1:
+        /*
+         * Find the first slot used. Remove the entry for the first
+         * table if the slot is not 0. For slot 0, the ID map was done
+         * in the second table.
+         */
+        lsr   x1, x19, #FIRST_SHIFT
+        and   x1, x1, #LPAE_ENTRY_MASK  /* x1 := first slot */
+        cbz   x1, 1f
+        /* It is not in slot 0, remove the entry */
+        ldr   x0, =boot_first           /* x0 := first table */
+        str   xzr, [x0, x1, lsl #3]
+        b     id_map_removed
+
+1:
+        /*
+         * Find the second slot used. Remove the entry for the first
+         * table if the slot is not 1 (runtime Xen mapping is 2M - 4M).
+         * For slot 1, it means the ID map was not created.
+         */
+        lsr   x1, x19, #SECOND_SHIFT
+        and   x1, x1, #LPAE_ENTRY_MASK  /* x1 := first slot */
+        cmp   x1, #1
+        beq   id_map_removed
+        /* It is not in slot 1, remove the entry */
+        ldr   x0, =boot_second          /* x0 := second table */
+        str   xzr, [x0, x1, lsl #3]
+
+id_map_removed:
+        /* See asm-arm/arm64/flushtlb.h for the explanation of the sequence. */
+        dsb   nshst
+        tlbi  alle2
+        dsb   nsh
+        isb
+
+        ret
+ENDPROC(remove_id_map)
+
 setup_fixmap:
-        /* Now we can install the fixmap and dtb mappings, since we
-         * don't need the 1:1 map any more */
-        dsb   sy
 #if defined(CONFIG_EARLY_PRINTK) /* Fixmap is only used by early printk */
         /* Add UART to the fixmap table */
         ldr   x1, =xen_fixmap        /* x1 := vaddr (xen_fixmap) */
@@ -653,19 +718,10 @@ setup_fixmap:
         ldr   x1, =FIXMAP_ADDR(0)
         lsr   x1, x1, #(SECOND_SHIFT - 3)   /* x1 := Slot for FIXMAP(0) */
         str   x2, [x4, x1]           /* Map it in the fixmap's slot */
-#endif
 
-        /*
-         * Flush the TLB in case the 1:1 mapping happens to clash with
-         * the virtual addresses used by the fixmap or DTB.
-         */
-        dsb   sy                     /* Ensure any page table updates made 
above
-                                      * have occurred. */
-
-        isb
-        tlbi  alle2
-        dsb   sy                     /* Ensure completion of TLB flush */
-        isb
+        /* Ensure any page table updates made above have occurred */
+        dsb   nshst
+#endif
         ret
 ENDPROC(setup_fixmap)
 
-- 
2.11.0


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