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Re: [PATCH v2 2/2] xen/arm: Add defensive barrier in get_cycles for Arm64

On 09/01/2021 12:16, Wei Chen wrote:
HI Julien,

Hi Wei,

-----Original Message-----
From: Julien Grall <julien@xxxxxxx>
Sent: 2021年1月8日 19:56
To: Wei Chen <Wei.Chen@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx;
Cc: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>; Penny Zheng
<Penny.Zheng@xxxxxxx>; Jiamei Xie <Jiamei.Xie@xxxxxxx>; nd
Subject: Re: [PATCH v2 2/2] xen/arm: Add defensive barrier in get_cycles for

Hi Wei,

On 08/01/2021 06:21, Wei Chen wrote:
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>
      Link: https://lore.kernel.org/linux-arm-
      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.

[1] https://lists.xenproject.org/archives/html/xen-devel/2020-
[2] https://lkml.org/lkml/2020/3/13/645

Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
v1 -> v2:
1. Update commit message to refer Linux commit.
2. Change u64 to uint64_t
   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)

OOI, is there any particular reason to create a new helper?

Yes, I try to reduce the #if defined(CONFIG_ARM_64) chunks.

Hmmm... There is no #ifdef chunk in read_cntpct_stable(). Did I miss anything?

I think
if we introduce an empty helper for Arm32, we can reduce the other
chunk inside get_cycles. In addition, if we need to do the same work
for Arm32 in the future, we may not need to modify get_cycles.
I don't really follow this. I wasn't asking about read_cntpct_enforce_ordering(). Instead I was asking about read_cntpct_stable() which looks like you just split get_cycles().

This change looks completely unrelated to the purpose of this series. I am not going to ask to split it, but I think this should be explained in the commit message.


Julien Grall



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