Another round of speculative-execution vulnerabilities [LWN.net]

Another round of speculative-execution vulnerabilities

Posted Aug 9, 2023 9:57 UTC (Wed) by malmedal (subscriber, #56172)
In reply to: Another round of speculative-execution vulnerabilities by Wol
Parent article: Another round of speculative-execution vulnerabilities

> because if the chip speed times the size of the computer is faster than the speed of light, well, things ain't going to work!

No, chip designers face many problems, but that one is solved. They are making complicated networks, called clock-trees, or meshes, that ensure each clock edge arrives at all components in the clock-domain simultaneously.

The problems they can't seem to solve include power and the fact that wires are getting slower faster than they get shorter in recent processes.

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Another round of speculative-execution vulnerabilities

Posted Aug 9, 2023 11:53 UTC (Wed) by Wol (subscriber, #4433) [Link] (5 responses)

> No, chip designers face many problems, but that one is solved. They are making complicated networks, called clock-trees, or meshes, that ensure each clock edge arrives at all components in the clock-domain simultaneously.

That's the wrong problem! Making sure all the signals arrive together is easy. The problem is that if your edges do not share a common "light cone", you're stuffed!

Increasing the frequency reduces the size of the light cone. If the light cone is to encompass the entire chip, then the upper limit on frequency is 5GHz. As others have said, if you're only interested in communicating internal to a single core, then LOCALLY you can increase the frequency further, because everything you're interested in fits into a smaller light cone.

You can have all the fancy clock-trees you like, but if your components are that far apart that you physically require faster-than-light information transfer, you're stuffed.

Cheers,
Wol

Another round of speculative-execution vulnerabilities

Posted Aug 9, 2023 12:34 UTC (Wed) by malmedal (subscriber, #56172) [Link] (3 responses)

Signal propagation speed in a wire is much slower than light speed. The chips are already much larger than the "light-cone". It gets progressively harder but there is no hard limit at a specific place.

The actual speed was about 1mm per nanosecond over ten years ago. It is probably less than 0.05mm/ns now.

Another round of speculative-execution vulnerabilities

Posted Aug 9, 2023 13:18 UTC (Wed) by Wol (subscriber, #4433) [Link] (2 responses)

So what you're saying is the speed of light slows down massively as the wire size shrinks. Ouch. Although I know the speed of electron propagation is much much slower. So it's a case of the fewer electrons fit on the wire, the closer the speed of light down the wire approximates to the speed of electrons in the wire? Painful!

Cheers,
Wol

Another round of speculative-execution vulnerabilities

Posted Aug 9, 2023 13:36 UTC (Wed) by malmedal (subscriber, #56172) [Link]

Hmmm, I've understood the issue to be explained by the telegraph equations: https://en.m.wikipedia.org/wiki/Telegrapher%27s_equations, but I suppose at the latest processes they may possibly need to account for individual elections. Which would be an extra complication.

Another round of speculative-execution vulnerabilities

Posted Aug 9, 2023 14:37 UTC (Wed) by joib (subscriber, #8541) [Link]

With my physicist pedantery hat on, no, I don't think that's exactly right. Now, my specialty was never high speed small scale circuits so I could talk out of my ass, but anyway; The speed of light in a conductor (that is, the velocity with which the electromagnetic wave propagates in the medium) is about 2/3 of the speed of light in vacuum, including for very narrow conductors. That is, very very fast. Similarly, the electron drift velocity also remains as it is for larger conductors, that is very very low, on the order of um or mm/s.

What changes then is that for these very small conductors you'll find on modern deep submicron integrated circuits, the relative capacitance of the conductor starts to rise (and the resistance doesn't go down as well as you'd like either). This leads to a phenomenon where when you apply a voltage on one end of the conductor, it takes longer until the voltage/current rises enough on the other end to be registered as a 0->1 flip. So in effect it appears as if the speed of signal propagation drops. I'm not sure how well the telegrapher's equations mentioned by malmedal in the sibling post applies to multi-GHz signals propagating in these very narrow conductors, but something like that is the gist of it. I don't think you need to apply quantum mechanics or study the behavior of individual electrons per se to understand this phenomena.

Another round of speculative-execution vulnerabilities

Posted Aug 9, 2023 22:41 UTC (Wed) by magnus (subscriber, #34778) [Link]

You can always add pipeline stages to transfer the data over several cycles to overcome this (at the cost of latency of course).