Author: Matthew Dillon <dil...@apollo.backplane.com>
Date: Wed Aug 9 22:20:52 2017 -0700
kernel - Lower VM_MAX_USER_ADDRESS to finalize work-around for Ryzen bug
* Reduce VM_MAX_USER_ADDRESS by 2MB, effectively making the top 2MB of the
user address space unmappable. The user stack now starts 2MB down from
where it did before. Theoretically we only need to reduce the top of
the user address space by 4KB, but doing it by 2MB may be more useful for
future page table optimizations.
* As per AMD, Ryzen has an issue when the instruction pre-fetcher crosses
from canonical to non-canonical address space. This can only occur at
the top of the user stack.
In DragonFlyBSD, the signal trampoline resides at the top of the user
and an IRETQ into it can cause a Ryzen box to lockup and destabilize due
to this action. The bug case was, basically two cpu threads on the same
core, one in a cpu-bound loop of some sort while the other takes a normal
UNIX signal (causing the IRETQ into the signal trampoline). The IRETQ
microcode freezes until the cpu-bound loop terminates, preventing the
cpu thread from being able to take any interrupt or IPI whatsoever for
the duration, and the cpu may destabilize afterwords as well.
* The pre-fetcher is somewhat heuristical, so just moving the trampoline
down is no guarantee if the top 4KB of the user stack is mapped or
It is better to make the boundary unmappable by userland.
* Bug first tracked down by myself in early 2017. AMD validated the bug
and determined that unmapping the boundary page completely solves the
* Also retain the code which places the signal trampoline in its own page
so we can maintain separate protection settings for the code, and make it
Summary of changes:
sys/cpu/x86_64/include/signal.h | 5 +++++
sys/kern/kern_exec.c | 10 ++--------
sys/platform/pc64/include/vmparam.h | 10 +++++++++-
sys/platform/vkernel64/include/vmparam.h | 9 ++++++++-
4 files changed, 24 insertions(+), 10 deletions(-)
DragonFly BSD source repository