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AR# 47546

Zynq-7000, APU - Processor Might Miss Watchpoint On Second Part Of Unaligned Access Crossing Page Boundary

Description

Under rare conditions, a watch-point on the second part of an unaligned access crossing a page boundary that misses in the microTLB for the second part of its request might not be detected. A guard watch-point can be set on the last byte of the first page and handle the false-positive matches when they occur.

Solution

Impact:

Minor. A watch-point should be triggered and it is not.

Work-around:

Set a guard watch-point on the last byte of the previous page, and software handles any "false positive" matches.

Configuration Affected:

Systems that use one or both of the processors.

Device Revision(s) Affected: All, no plans to fix. Refer to (Xilinx Answer 47916) - Zynq-7000 Design Advisory Master Answer Record


When a watch-point is set on any of the first 3 bytes of a 4 KB memory region, and unaligned accesses are not being faulted, then the issue can happen.

The issue requires a previous conditional instruction which accesses the second 4KB memory region (= where the watch-point is set), misses in the microTLB, and its condition has failed. The issue also requires that no other microTLB miss occurs between this conditional failed instruction and the unaligned access, which implies that the unaligned access must hit in the microTLB for the first part of its access.
AR# 47546
Date Created 05/24/2012
Last Updated 08/06/2012
Status Active
Type Design Advisory
Devices
  • Zynq-7000