[PATCH v4 01/14] xen/arm64: flushtlb: Reduce scope of barrier for local TLB flush

[Julien Grall <julien@xxxxxxx>]

From: Julien Grall <jgrall@xxxxxxxxxx>

Per D5-4929 in ARM DDI 0487H.a:
"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 means barrier after local TLB flushes could be reduced to
non-shareable.

Note that the scope of the barrier in the workaround has not been
changed because Linux v6.1-rc8 is also using 'ish' and I couldn't
find anything in the Neoverse N1 suggesting that a 'nsh' would
be sufficient.

Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>

----

    I have used an older version of the Arm Arm because the explanation
    in the latest (ARM DDI 0487I.a) is less obvious. I reckon the paragraph
    about DSB in D8.13.8 is missing the shareability. But this is implied
    in B2.3.11:

    "If the required access types of the DSB is reads and writes, the
     following instructions issued by PEe before the DSB are complete for
     the required shareability domain:

     [...]

     — All TLB maintenance instructions.
    "

    Changes in v4:
        - Fix typoes
        - Don't modify the shareability in the workaround
        - Add Michal's reviewed-by tag

    Changes in v3:
        - Patch added
---
 xen/arch/arm/include/asm/arm64/flushtlb.h | 25 ++++++++++++++---------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/xen/arch/arm/include/asm/arm64/flushtlb.h 
b/xen/arch/arm/include/asm/arm64/flushtlb.h
index 7c5431518741..a40693b08dd3 100644
--- a/xen/arch/arm/include/asm/arm64/flushtlb.h
+++ b/xen/arch/arm/include/asm/arm64/flushtlb.h
@@ -12,8 +12,9 @@
  * ARM64_WORKAROUND_REPEAT_TLBI:
  * Modification of the translation table for a virtual address might lead to
  * read-after-read ordering violation.
- * The workaround repeats TLBI+DSB operation for all the TLB flush operations.
- * While this is stricly not necessary, we don't want to take any risk.
+ * The workaround repeats TLBI+DSB ISH operation for all the TLB flush
+ * operations. While this is strictly not necessary, we don't want to
+ * take any risk.
  *
  * For Xen page-tables the ISB will discard any instructions fetched
  * from the old mappings.
@@ -21,12 +22,16 @@
  * For the Stage-2 page-tables the ISB ensures the completion of the DSB
  * (and therefore the TLB invalidation) before continuing. So we know
  * the TLBs cannot contain an entry for a mapping we may have removed.
+ *
+ * Note that for local TLB flush, using non-shareable (nsh) is sufficient
+ * (see D5-4929 in ARM DDI 0487H.a). Although, the memory barrier in
+ * for the workaround is left as inner-shareable to match with Linux.
  */
-#define TLB_HELPER(name, tlbop)                  \
+#define TLB_HELPER(name, tlbop, sh)              \
 static inline void name(void)                    \
 {                                                \
     asm volatile(                                \
-        "dsb  ishst;"                            \
+        "dsb  "  # sh  "st;"                     \
         "tlbi "  # tlbop  ";"                    \
         ALTERNATIVE(                             \
             "nop; nop;",                         \
@@ -34,25 +39,25 @@ static inline void name(void)                    \
             "tlbi "  # tlbop  ";",               \
             ARM64_WORKAROUND_REPEAT_TLBI,        \
             CONFIG_ARM64_WORKAROUND_REPEAT_TLBI) \
-        "dsb  ish;"                              \
+        "dsb  "  # sh  ";"                       \
         "isb;"                                   \
         : : : "memory");                         \
 }
 
 /* Flush local TLBs, current VMID only. */
-TLB_HELPER(flush_guest_tlb_local, vmalls12e1);
+TLB_HELPER(flush_guest_tlb_local, vmalls12e1, nsh);
 
 /* Flush innershareable TLBs, current VMID only */
-TLB_HELPER(flush_guest_tlb, vmalls12e1is);
+TLB_HELPER(flush_guest_tlb, vmalls12e1is, ish);
 
 /* Flush local TLBs, all VMIDs, non-hypervisor mode */
-TLB_HELPER(flush_all_guests_tlb_local, alle1);
+TLB_HELPER(flush_all_guests_tlb_local, alle1, nsh);
 
 /* Flush innershareable TLBs, all VMIDs, non-hypervisor mode */
-TLB_HELPER(flush_all_guests_tlb, alle1is);
+TLB_HELPER(flush_all_guests_tlb, alle1is, ish);
 
 /* Flush all hypervisor mappings from the TLB of the local processor. */
-TLB_HELPER(flush_xen_tlb_local, alle2);
+TLB_HELPER(flush_xen_tlb_local, alle2, nsh);
 
 /* Flush TLB of local processor for address va. */
 static inline void  __flush_xen_tlb_one_local(vaddr_t va)
-- 
2.38.1