Funny you should ask. The short answer is no. No VAX ever did
speculative or out of order execution.
Three developments had to come together for Spectre-style bugs:
1. Speculative execution that affects the cache or other indirectly
testable state.
2. A high-precision, user-mode timer (for measuring cache or branch
table perturbation effects).
3. A 'close-in' entity that can do low-latency measurements - the other
thread(s) in a hyper-threaded CPU; another core in a multi-core chip;
another processor on a SMP bus with extremely fast response time.
The VAX lacked all three. Aquarius and NVAX pipelined instruction
execution, but they did not go out of order. The high-precision timers
were IPRs only accessible in kernel mode. And the other processors in an
SMP were very 'distant' in modern terms.
While IBM gets the credit (or blame) for speculative execution, it was
Intel's decision to allow speculation to perturb the data cache that put
the cat among the pigeons. Tthe high-precision user mode timer started
with Alpha's cycle timer. As for close-in entities... the roots are
tangled. Hyperthreading was in the air in the mid to late 90s; EV8 was a
four-way threaded design. So was multicore - the WRL chip team proposed
an eight-core EV56 as a cheaper-to-design-and-build alternative to EV8.
A more interesting question - and one more relevant, since a lot of
Alphas are still running - is whether Alpha (in particular, EV6) is
vulnerable. Again, I'm pretty sure the answer is no. While EV6 did
speculative execution, it would not allow speculative loads or stores to
perturb the cache. It would be very difficult to exploit the branch
prediction or subroutine return prediction tables, because they would be
massively perturbed by any switch to a measurement thread or process.
And theAlpha interprocessor SMP buses were too slow to allow effective
measurements by a different processor.
/Bob
On 9/17/2019 12:00 PM, [email protected] wrote:
Message: 1
Date: Tue, 17 Sep 2019 09:55:01 -0400
From: Paul Koning<[email protected]>
To: SIMH<[email protected]>, "General Discussion: On-Topic and
Off-Topic Posts"<[email protected]>
Subject: [Simh] Fwd: VAX + Spectre
Message-ID:<[email protected]>
Content-Type: text/plain; charset="utf-8"
"Spectre" is one of two notorious bugs of modern CPUs involving speculative
execution. I rather doubt that VAX is affected by this but I suspect others here have a
lot more knowledge.
paul
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