xen/arm: Add defensive barrier in get_cycles for Arm64

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@vger.kernel.org>
    Cc: Marc Zyngier <marc.zyngier@arm.com>
    Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/linux-arm-kernel/alpine.DEB.2.21.1902081950260.1662@nanos.tec.linutronix.de/
    Reported-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Will Deacon <will.deacon@arm.com>
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
easily.

[1] https://lists.xenproject.org/archives/html/xen-devel/2020-12/msg00181.html
[2] https://lkml.org/lkml/2020/3/13/645

Signed-off-by: Wei Chen <wei.chen@arm.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>