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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH v4 1/8] 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 were adopted.
Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
---
It is very likely we will need to re-introduce the 1:1 mapping to cater
secondary CPUs boot and suspend/resume. For now, the attempt is to make
boot CPU path fully Arm Arm compliant.
Changes in v4:
- Fix typo
- Remove unnecessary comments
- Update the commit message to mention the difference between
the sequence described in tlbflush.h and the one used in the
code.
Changes in v3:
- Avoid hardcoding slots
Changes in v2:
- s/ID map/1:1 mapping/
- Rename remove_id_map() to remove_identity_mapping()
- Add missing signed-off-by
---
xen/arch/arm/arm64/head.S | 90 +++++++++++++++++++++++++++++++++++++++--------
1 file changed, 75 insertions(+), 15 deletions(-)
diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index ba24b05fa2..4c9a69be63 100644
--- 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)
--
2.11.0
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