[RFC PATCH v2 0/2] x86/speculation: Add finer control for when to issue IBPB

From: Anand K Mistry
Date: Thu Apr 29 2021 - 04:45:07 EST

It is documented in Documentation/admin-guide/hw-vuln/spectre.rst, that
disabling indirect branch speculation for a user-space process creates
more overhead and cause it to run slower. The performance hit varies by
CPU, but on the AMD A4-9120C and A6-9220C CPUs, a simple ping-pong using
pipes between two processes runs ~10x slower when disabling IB

Patch 2, included in this RFC but not intended for commit, is a simple
program that demonstrates this issue. Running on a A4-9120C without IB
speculation disabled, each process ping-pong takes ~7us:
localhost ~ # taskset 1 /usr/local/bin/test
iters: 262144, t: 1936300, iter/sec: 135383, us/iter: 7

But when IB speculation is disabled, that number increases
localhost ~ # taskset 1 /usr/local/bin/test d
iters: 16384, t: 1500518, iter/sec: 10918, us/iter: 91

Although this test is a worst-case scenario, we can also consider a real
situation: an audio server (i.e. pulse). If we imagine a low-latency
capture, with 10ms packets and a concurrent task on the same CPU (i.e.
video encoding, for a video call), the audio server will preempt the
CPU at a rate of 100HZ. At 91us overhead per preemption (switching to
and from the audio process), that's 0.9% overhead for one process doing
preemption. In real-world testing (on a A4-9120C), I've seen 9% of CPU
used by IBPB when doing a 2-person video call.

With this patch, the number of IBPBs issued can be reduced to the
minimum necessary, only when there's a potential attacker->victim
process switch.

Running on the same A4-9120C device, this patch reduces the performance
hit of IBPB by ~half, as expected:
localhost ~ # taskset 1 /usr/local/bin/test ds
iters: 32768, t: 1824043, iter/sec: 17964, us/iter: 55

It should be noted, CPUs from multiple vendors experience a performance
hit due to IBPB. I also tested a Intel i3-8130U which sees a noticable
(~2x) increase in process switch time due to IBPB.
IB spec enabled:
localhost ~ # taskset 1 /usr/local/bin/test
iters: 262144, t: 1210821us, iter/sec: 216501, us/iter: 4

IB spec disabled:
localhost ~ # taskset 1 /usr/local/bin/test d
iters: 131072, t: 1257583us, iter/sec: 104225, us/iter: 9

Open questions:
- There are a significant number of task flags, which also now reaches the
limit of the 'long' on 32-bit systems. Should the 'mode' flags be
stored somewhere else?
- Having x86-specific flags in linux/sched.h feels wrong. However, this
is the mechanism for doing atomic flag updates. Is there an alternate

Open tasks:
- Documentation
- Naming

Changes in v2:
- Make flag per-process using prctl().

Anand K Mistry (2):
x86/speculation: Allow per-process control of when to issue IBPB
selftests: Benchmark for the cost of disabling IB speculation

arch/x86/include/asm/thread_info.h | 4 +
arch/x86/kernel/cpu/bugs.c | 56 +++++++++
arch/x86/kernel/process.c | 10 ++
arch/x86/mm/tlb.c | 51 ++++++--
include/linux/sched.h | 10 ++
include/uapi/linux/prctl.h | 5 +
.../testing/selftests/ib_spec/ib_spec_bench.c | 109 ++++++++++++++++++
7 files changed, 236 insertions(+), 9 deletions(-)
create mode 100644 tools/testing/selftests/ib_spec/ib_spec_bench.c