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Re: [Xen-devel] [PATCH v3 02/28] xen/arm64: head: Remove 1:1 mapping as soon as it is not used

Hi Stefano,

On 8/22/19 6:58 PM, Stefano Stabellini wrote:

On Mon, 12 Aug 2019, Julien Grall wrote:

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.

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 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 | 94 +++++++++++++++++++++++++++++++++++++++--------
  1 file changed, 79 insertions(+), 15 deletions(-)

diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index 50cff08756..ec138aae3e 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 
@@ -301,6 +306,13 @@ 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. */
@@ -626,10 +638,71 @@ enable_mmu:
          ret
  ENDPROC(enable_mmu)
+/* 
+ * Remove the 1:1 map for the page-tables. It is not easy to keep track

                          ^ from


I will fix it in the next version.




+ * 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.


This part of the comment is good



+         * For slot XEN_ZEROETH_SLOT, the 1:1 mapping was either done in first
+         * or second table.


I don't think this sentence is very useful now that the slot is not
hard-coded anymore. I would remove it. Instead, I would write something
like "The slot XEN_ZEROETH_SLOT is used for the XEN_VIRT_START mapping."
The same goes for all the other levels.
I think this is a bit confusing because one may think the XEN_VIRT_START mapping is using the full slot. Whereas it is only partially using it. 

So I would prefer to drop the sentence completely.





+         */
+        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. For slot XEN_FIRST_SLOT,
+         * the 1:1 mapping was done in the second table.
+         */
+        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. For slot XEN_SECOND_SLOT,
+         * it means the 1:1 mapping was not created.
+         */
+        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


I just want to point out that asm-arm/arm64/flushtlb.h says to use:

  * DSB ISHST        // Ensure prior page-tables updates have completed
  * TLBI...          // Invalidate the TLB
  * DSB ISH          // Ensure the TLB invalidation has completed
  * ISB              // See explanation below

Also implemented as:

     asm volatile(               \
         "dsb  ishst;"           \
         "tlbi "  # tlbop  ";"   \
         "dsb  ish;"             \
         "isb;"                  \
         : : : "memory");        \

Why is non-shareable enough? Shouldn't it be inner shareable?
I thought I answered this before. I should have probably clarified in the commit message.
nsh is used (rather than ish) because the TLB flush is local (see page D5-230 ARM DDI 0487D.a). For convenience here is the text: 

"In all cases in this section where a DMB or DSB is referred to, it
refers to a DMB or DSB whose required access type is
both loads and stores. A DSB NSH is sufficient to ensure completion of
TLB maintenance instructions that apply to a
single PE. A DSB ISH is sufficient to ensure completion of TLB
maintenance instructions that apply to PEs in the
same Inner Shareable domain."
This is something Linux already does but I wasn't able to find the proper justification in the Arm Arm. So I chose a more conservative approach that is explained in section K11.5.3 (ARM DDI 0487D.a).
I have an action to update tlbflush.h before this is in a huge pile of cleanup/optimization. 

Cheers,

--
Julien Grall

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