xen/arm32: head: Remove 1:1 mapping as soon as it is not used

The 1:1 mapping 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 1:1 mapping can be anywhere in the memory, it is easier to remove
all the entries added as soon as the 1:1 mapping is not used rather than
adding the Break-Before-Make sequence everywhere.

It is difficult to track where exactly the 1:1 mapping was created
without a full rework of create_page_tables(). Instead, introduce a new
function remove_identity_mapping() will look where is the top-level entry
for the 1:1 mapping 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 1:1 mapping. Note that this still doesn't make the Secondary
CPUs path safe but it is not making it worst.

Note that the TLB flush sequence is same sequence as described in
asm-arm/arm32/flushtlb.h with a twist. Per G5-5532 ARM DDI 0487D.a,
a dsb nsh is sufficient for local flushed. Note the section is from the
AArch32 Armv8 spec, I wasn't able to find the same exact section in the
Armv7 spec but this is dotted as local operations only applies to
non-shareable domain. This was missed while reworking the header and
therefore a more conservative way was adopted.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/arm32/head.S

@@ -32,6 +32,10 @@
 #define PT_UPPER(x) (PT_##x & 0xf00)
 #define PT_LOWER(x) (PT_##x & 0x0ff)
 
+/* Convenience defines to get slot used by Xen mapping. */
+#define XEN_FIRST_SLOT      first_table_offset(XEN_VIRT_START)
+#define XEN_SECOND_SLOT     second_table_offset(XEN_VIRT_START)
+
 #if (defined (CONFIG_EARLY_PRINTK)) && (defined (EARLY_PRINTK_INC))
 #include EARLY_PRINTK_INC
 #endif
@@ -157,6 +161,13 @@ past_zImage:
         ldr   r0, =primary_switched
         mov   pc, r0
 primary_switched:
+        /*
+         * The 1:1 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_identity_mapping
         bl    setup_fixmap
 #ifdef CONFIG_EARLY_PRINTK
         /* Use a virtual address to access the UART. */
@@ -481,12 +492,61 @@ enable_mmu:
         mov   pc, lr
 ENDPROC(enable_mmu)
 
-setup_fixmap:
+/*
+ * Remove the 1:1 map from the page-tables. It is not easy to keep track
+ * where the 1:1 map was mapped, so we will look for the top-level entry
+ * exclusive to the 1:1 map and remove it.
+ *
+ * Inputs:
+ *   r9 : paddr(start)
+ *
+ * Clobbers r0 - r3
+ */
+remove_identity_mapping:
+        /* r2:r3 := invalid page-table entry */
+        mov   r2, #0x0
+        mov   r3, #0x0
         /*
-         * Now we can install the fixmap and dtb mappings, since we
-         * don't need the 1:1 map any more
+         * Find the first slot used. Remove the entry for the first
+         * table if the slot is not XEN_FIRST_SLOT.
          */
-        dsb
+        lsr   r1, r9, #FIRST_SHIFT
+        mov_w r0, LPAE_ENTRY_MASK
+        and   r1, r1, r0              /* r1 := first slot */
+        cmp   r1, #XEN_FIRST_SLOT
+        beq   1f
+        /* It is not in slot 0, remove the entry */
+        ldr   r0, =boot_pgtable      /* r0 := root table */
+        lsl   r1, r1, #3             /* r1 := Slot offset */
+        strd  r2, r3, [r0, r1]
+        b     identity_mapping_removed
+
+1:
+        /*
+         * Find the second slot used. Remove the entry for the first
+         * table if the slot is not XEN_SECOND_SLOT.
+         */
+        lsr   r1, r9, #SECOND_SHIFT
+        mov_w r0, LPAE_ENTRY_MASK
+        and   r1, r1, r0             /* r1 := second slot */
+        cmp   r1, #XEN_SECOND_SLOT
+        beq   identity_mapping_removed
+        /* It is not in slot 1, remove the entry */
+        ldr   r0, =boot_second       /* r0 := second table */
+        lsl   r1, r1, #3             /* r1 := Slot offset */
+        strd  r2, r3, [r0, r1]
+
+identity_mapping_removed:
+        /* See asm-arm/arm32/flushtlb.h for the explanation of the sequence. */
+        dsb   nshst
+        mcr   CP32(r0, TLBIALLH)
+        dsb   nsh
+        isb
+
+        mov   pc, lr
+ENDPROC(remove_identity_mapping)
+
+setup_fixmap:
 #if defined(CONFIG_EARLY_PRINTK) /* Fixmap is only used by early printk */
         /* Add UART to the fixmap table */
         ldr   r1, =xen_fixmap        /* r1 := vaddr (xen_fixmap) */
@@ -496,7 +556,6 @@ setup_fixmap:
         orr   r2, r2, #PT_LOWER(DEV_L3) /* r2:r3 := 4K dev map including UART */
         mov   r3, #0x0
         strd  r2, r3, [r1, #(FIXMAP_CONSOLE*8)] /* Map it in the first fixmap's slot */
-1:
 
         /* Map fixmap into boot_second */
         ldr   r1, =boot_second       /* r1 := vaddr (boot_second) */
@@ -508,19 +567,10 @@ setup_fixmap:
         mov   r4, r4, lsr #(SECOND_SHIFT - 3)   /* r4 := Slot for FIXMAP(0) */
         mov   r3, #0x0
         strd  r2, r3, [r1, r4]       /* Map it in the fixmap's slot */
+
+        /* Ensure any page table updates made above have occurred. */
+        dsb   nshst
 #endif
-
-        /*
-         * Flush the TLB in case the 1:1 mapping happens to clash with
-         * the virtual addresses used by the fixmap or DTB.
-         */
-        dsb                          /* Ensure any page table updates made above
-                                      * have occurred. */
-
-        isb
-        mcr   CP32(r0, TLBIALLH)     /* Flush hypervisor TLB */
-        dsb                          /* Ensure completion of TLB flush */
-        isb
         mov   pc, lr
 ENDPROC(setup_fixmap)