xen/arm64: 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 D5-2530 ARM DDI 0487D.a,
a dsb nsh is sufficient for local flush. This part of the Arm Arm
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>

diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index ba24b05fa2a0490a6952c69115381040347f2789..4c9a69be6336d2be918324ef05f968a3e5ca9629 100644 (file)
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -33,6 +33,11 @@
 #define PT_DEV    0xe71 /* nG=1 AF=1 SH=10 AP=01 NS=1 ATTR=100 T=0 P=1 */
 #define PT_DEV_L3 0xe73 /* nG=1 AF=1 SH=10 AP=01 NS=1 ATTR=100 T=1 P=1 */
 
+/* Convenience defines to get slot used by Xen mapping. */
+#define XEN_ZEROETH_SLOT    zeroeth_table_offset(XEN_VIRT_START)
+#define XEN_FIRST_SLOT      first_table_offset(XEN_VIRT_START)
+#define XEN_SECOND_SLOT     second_table_offset(XEN_VIRT_START)
+
 #define __HEAD_FLAG_PAGE_SIZE   ((PAGE_SHIFT - 10) / 2)
 
 #define __HEAD_FLAG_PHYS_BASE   1
@@ -312,6 +317,13 @@ real_start_efi:
         ldr   x0, =primary_switched
         br    x0
 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. */
@@ -648,10 +660,67 @@ enable_mmu:
         ret
 ENDPROC(enable_mmu)
 
+/*
+ * 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:
+ *   x19: paddr(start)
+ *
+ * Clobbers x0 - x1
+ */
+remove_identity_mapping:
+        /*
+         * Find the zeroeth slot used. Remove the entry from zeroeth
+         * table if the slot is not XEN_ZEROETH_SLOT.
+         */
+        lsr   x1, x19, #ZEROETH_SHIFT   /* x1 := zeroeth slot */
+        cmp   x1, #XEN_ZEROETH_SLOT
+        beq   1f
+        /* It is not in slot XEN_ZEROETH_SLOT, remove the entry. */
+        ldr   x0, =boot_pgtable         /* x0 := root table */
+        str   xzr, [x0, x1, lsl #3]
+        b     identity_mapping_removed
+
+1:
+        /*
+         * Find the first slot used. Remove the entry for the first
+         * table if the slot is not XEN_FIRST_SLOT.
+         */
+        lsr   x1, x19, #FIRST_SHIFT
+        and   x1, x1, #LPAE_ENTRY_MASK  /* x1 := first slot */
+        cmp   x1, #XEN_FIRST_SLOT
+        beq   1f
+        /* It is not in slot XEN_FIRST_SLOT, remove the entry. */
+        ldr   x0, =boot_first           /* x0 := first table */
+        str   xzr, [x0, x1, lsl #3]
+        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   x1, x19, #SECOND_SHIFT
+        and   x1, x1, #LPAE_ENTRY_MASK  /* x1 := first slot */
+        cmp   x1, #XEN_SECOND_SLOT
+        beq   identity_mapping_removed
+        /* It is not in slot 1, remove the entry */
+        ldr   x0, =boot_second          /* x0 := second table */
+        str   xzr, [x0, x1, lsl #3]
+
+identity_mapping_removed:
+        /* See asm-arm/arm64/flushtlb.h for the explanation of the sequence. */
+        dsb   nshst
+        tlbi  alle2
+        dsb   nsh
+        isb
+
+        ret
+ENDPROC(remove_identity_mapping)
+
 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) */
@@ -669,19 +738,10 @@ 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)