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[xen staging] xen/arm: Add defensive barrier in get_cycles for Arm64

commit 7936671da9fbf645d6bb207608f7b81c27f992de
Author:     Wei Chen <wei.chen@xxxxxxx>
AuthorDate: Fri Jan 8 14:21:26 2021 +0800
Commit:     Julien Grall <jgrall@xxxxxxxxxx>
CommitDate: Wed Jan 20 17:26:18 2021 +0000

    xen/arm: Add defensive barrier in get_cycles for Arm64
    Per the discussion [1] on the mailing list, we'd better to
    have a barrier after reading CNTPCT in get_cycles. If there
    is not any barrier there. When get_cycles being used in some
    seqlock critical context in the future, the seqlock can be
    speculated potentially.
    We import Linux commit 75a19a0202db21638a1c2b424afb867e1f9a2376:
        arm64: arch_timer: Ensure counter register reads occur with seqlock held
        When executing clock_gettime(), either in the vDSO or via a system call,
        we need to ensure that the read of the counter register occurs within
        the seqlock reader critical section. This ensures that updates to the
        clocksource parameters (e.g. the multiplier) are consistent with the
        counter value and therefore avoids the situation where time appears to
        go backwards across multiple reads.
        Extend the vDSO logic so that the seqlock critical section covers the
        read of the counter register as well as accesses to the data page. Since
        reads of the counter system registers are not ordered by memory barrier
        instructions, introduce dependency ordering from the counter read to a
        subsequent memory access so that the seqlock memory barriers apply to
        the counter access in both the vDSO and the system call paths.
        Cc: <stable@xxxxxxxxxxxxxxx>
        Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
        Tested-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
        Reported-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
        Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
    While we are not aware of such use in Xen, it would be best to add the
    barrier to avoid any suprise.
    In order to reduce the impact of new barrier, we perfer to
    use enforce order instead of ISB [2].
    Currently, enforce order is not applied to arm32 as this is
    not done in Linux at the date of this patch. If this is done
    in Linux it will need to be also done in Xen.
    To avoid adding read_cntpct_enforce_ordering everywhere, we introduced
    a new helper read_cntpct_stable to replace original get_cycles, and turn
    get_cycles to a wrapper which we can add read_cntpct_enforce_ordering
    [2] https://lkml.org/lkml/2020/3/13/645
    Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
    Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>
 xen/include/asm-arm/time.h | 43 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 41 insertions(+), 2 deletions(-)

diff --git a/xen/include/asm-arm/time.h b/xen/include/asm-arm/time.h
index 5c4529ebb5..6b8fd839dd 100644
--- a/xen/include/asm-arm/time.h
+++ b/xen/include/asm-arm/time.h
@@ -11,9 +11,26 @@
 typedef uint64_t cycles_t;
-static inline cycles_t get_cycles(void)
+ * Ensure that reads of the counter are treated the same as memory reads
+ * for the purposes of ordering by subsequent memory barriers.
+ */
+#if defined(CONFIG_ARM_64)
+#define read_cntpct_enforce_ordering(val) do { \
+    uint64_t tmp, _val = (val);                \
+                                               \
+    asm volatile(                              \
+    "eor %0, %1, %1\n"                         \
+    "add %0, sp, %0\n"                         \
+    "ldr xzr, [%0]"                            \
+    : "=r" (tmp) : "r" (_val));                \
+} while (0)
+#define read_cntpct_enforce_ordering(val) do {} while (0)
+static inline cycles_t read_cntpct_stable(void)
-    isb();
      * ARM_WORKAROUND_858921: Cortex-A73 (all versions) counter read
      * can return a wrong value when the counter crosses a 32bit boundary.
@@ -36,6 +53,28 @@ static inline cycles_t get_cycles(void)
+static inline cycles_t get_cycles(void)
+    cycles_t cnt;
+    isb();
+    cnt = read_cntpct_stable();
+    /*
+     * If there is not any barrier here. When get_cycles being used in
+     * some seqlock critical context in the future, the seqlock can be
+     * speculated potentially.
+     *
+     * To prevent seqlock from being speculated silently, we add a barrier
+     * here defensively. Normally, we just need an ISB here is enough, but
+     * considering the minimum performance cost. We prefer to use enforce
+     * order here.
+     */
+    read_cntpct_enforce_ordering(cnt);
+    return cnt;
 /* List of timer's IRQ */
 enum timer_ppi
generated by git-patchbot for /home/xen/git/xen.git#staging



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