In the early days of personal computing CPU bugs were so rare as to be newsworthy.
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In the early days of personal computing CPU bugs were so rare as to be newsworthy. The infamous Pentium FDIV bug is remembered by many, and even earlier CPUs had their own issues (the 6502 comes to mind). Nowadays they've become so common that I encounter them routinely while triaging crash reports sent from Firefox users. Given the nature of CPUs you might wonder how these bugs arise, how they manifest and what can and can't be done about them. 🧵 1/31
@gabrielesvelto Fascinating thread, especially the degradation over time inherit to modern processors. That came up recently in an interesting viral video on a world where we forget how to make new CPUs.
Bit of an aside, but I assume this affects other architectures? The thread mentioned Intel and AMD, but I assume Arm and Risc-V are similarly prone to these sorts of problems?
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Bonus end-of-thread post: when you encounter these bugs try to cut the hardware designers some slack. They work on increasingly complex stuff, with increasingly pressing deadlines and under upper management who rarely understands what they're doing. Put the blame for these bugs where it's due: on executives that haven't allocated enough time, people and resources to make a quality product.
@gabrielesvelto that's the deep nerdy stuff I love about IT! Thanks a ton for sharing this!
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The speed at which signals propagate in circuits is proportional to how much voltage is being applied. In older CPUs this voltage was fixed, but in modern ones it changes thousands of times per second to save power. Providing just as little voltage needed for a certain clock frequency can dramatically reduce power consumption, but providing too little voltage may cause a signal to arrive late, or the wrong signal to reach the pipeline register, causing in turn a cascade of failures. 24/31
@gabrielesvelto nitpick: the propagation velocity of a *signal* in a circuit is not affected by the voltage magnitude; that is a function of the (innate) dielectric constant of the material.
however, a higher core voltage does mean that a rising edge tends to reach the gate threshold voltage of a transistor more quickly, which reduces the time it takes for each asynchronous logic element's output to reach a well-defined state after a change in input, thus propagating logic *state* more quickly.
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@gabrielesvelto nitpick: the propagation velocity of a *signal* in a circuit is not affected by the voltage magnitude; that is a function of the (innate) dielectric constant of the material.
however, a higher core voltage does mean that a rising edge tends to reach the gate threshold voltage of a transistor more quickly, which reduces the time it takes for each asynchronous logic element's output to reach a well-defined state after a change in input, thus propagating logic *state* more quickly.
@gabrielesvelto (what you said is absolutely correct regarding "signals" in the HDL sense of the word, it just gets a bit muddled when we're simultaneously talking about the analogue behaviours of the actual electrical signals, hence the clarification ^^)
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Bonus end-of-thread post: when you encounter these bugs try to cut the hardware designers some slack. They work on increasingly complex stuff, with increasingly pressing deadlines and under upper management who rarely understands what they're doing. Put the blame for these bugs where it's due: on executives that haven't allocated enough time, people and resources to make a quality product.
@gabrielesvelto This was a phenomenal write-up, thank you!
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In the early days of personal computing CPU bugs were so rare as to be newsworthy. The infamous Pentium FDIV bug is remembered by many, and even earlier CPUs had their own issues (the 6502 comes to mind). Nowadays they've become so common that I encounter them routinely while triaging crash reports sent from Firefox users. Given the nature of CPUs you might wonder how these bugs arise, how they manifest and what can and can't be done about them. 🧵 1/31
@gabrielesvelto fantastic thread thank you :D
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In the early days of personal computing CPU bugs were so rare as to be newsworthy. The infamous Pentium FDIV bug is remembered by many, and even earlier CPUs had their own issues (the 6502 comes to mind). Nowadays they've become so common that I encounter them routinely while triaging crash reports sent from Firefox users. Given the nature of CPUs you might wonder how these bugs arise, how they manifest and what can and can't be done about them. 🧵 1/31
@gabrielesvelto Nice thread!
You seem to imply that bugs have become considerably more frequent, largely due to the increased complexity. Right?
To me it's not obvious that the larger number of known issues isn't to a large degree due to much better visibility (we didn't have anywhere close to today's automatic crash collection systems in the past) and due to the vastly increased number of CPUs... Do you have any gut feeling about that?
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@gabrielesvelto (what you said is absolutely correct regarding "signals" in the HDL sense of the word, it just gets a bit muddled when we're simultaneously talking about the analogue behaviours of the actual electrical signals, hence the clarification ^^)
@gsuberland thanks, I was playing a bit fast and loose with the terminology. As I was writing these toots I reminded myself that entire books have been written just to model transistor behavior and propagation delay, and my very crude wording would probably give their authors a heart attack.
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@gabrielesvelto Nice thread!
You seem to imply that bugs have become considerably more frequent, largely due to the increased complexity. Right?
To me it's not obvious that the larger number of known issues isn't to a large degree due to much better visibility (we didn't have anywhere close to today's automatic crash collection systems in the past) and due to the vastly increased number of CPUs... Do you have any gut feeling about that?
@AndresFreundTec I've been in charge of Firefox stability for ten years now and some of my early work to detect hardware issues dates back then. In pre-2020 years we could get a 2-3 bugs per year, usually across different CPUs. Now we get dozens, it's really on another level.
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@AndresFreundTec I've been in charge of Firefox stability for ten years now and some of my early work to detect hardware issues dates back then. In pre-2020 years we could get a 2-3 bugs per year, usually across different CPUs. Now we get dozens, it's really on another level.
@AndresFreundTec admittedly we get a lot more after a new microarchitecture launches, and then they go down as microcode updates get rolled out. If Microsoft hadn't started shipping microcode updates with their OS updates we'd be swamped.
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In the early days of personal computing CPU bugs were so rare as to be newsworthy. The infamous Pentium FDIV bug is remembered by many, and even earlier CPUs had their own issues (the 6502 comes to mind). Nowadays they've become so common that I encounter them routinely while triaging crash reports sent from Firefox users. Given the nature of CPUs you might wonder how these bugs arise, how they manifest and what can and can't be done about them. 🧵 1/31
@gabrielesvelto
There’s also meta-stability. If a value is snapshotted half way through it changing, it may occasionally result in the output not being one or zero, but some ‘half’ value. Depending on the circuits using that result, it may be interpreted as either 1 or 0 — and maybe different parts of the circuit will use different interpretations. Such intermediate states are only meta-stable, and will flip to a firm 1 or 0 at some indeterminate time later, possibly propagating the problem. -
@gabrielesvelto
There’s also meta-stability. If a value is snapshotted half way through it changing, it may occasionally result in the output not being one or zero, but some ‘half’ value. Depending on the circuits using that result, it may be interpreted as either 1 or 0 — and maybe different parts of the circuit will use different interpretations. Such intermediate states are only meta-stable, and will flip to a firm 1 or 0 at some indeterminate time later, possibly propagating the problem.@KimSJ ah yes, very good point. It's been a while since my days in hardware land and I had forgotten about it.
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@gabrielesvelto fantastic thread thank you :D
@dubiousblur glad you liked it!
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@gabrielesvelto Fascinating thread, especially the degradation over time inherit to modern processors. That came up recently in an interesting viral video on a world where we forget how to make new CPUs.
Bit of an aside, but I assume this affects other architectures? The thread mentioned Intel and AMD, but I assume Arm and Risc-V are similarly prone to these sorts of problems?
@tehstu yes, absolutely. I've encountered several bugs in AMD CPUs, not many on ARM just yet, but our ARM user-base is very small compared to x86, so it's just less likely for us to stumble upon them. Plus we have some machinery that can detect some hardware bugs automatically but it doesn't work on ARM just yet.
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However not all bugs can be fixed this way. Bugs within logic that sits on a critical path can rarely be fixed. Additionally some microcode fixes can only be made to work if the microcode is loaded at boot time, right when the CPU is initialized. If the updated microcode is loaded by the operating system it might be too late to reconfigure the core's operation, you'll need an updated UEFI firmware for some fix to work. 20/31
@gabrielesvelto but UEFI is already quite complex, it has to find block devices, read their partition tables, read FAT file systems, read directories and files, load data in memory and transfer execution. Wouldn't a patch after all that not be too late?
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I can't be sure that this is exactly what's happening on Raptor Lake CPUs, it's just a theory. But a modern CPU core has millions upon millions of these types of circuits, and a timing issue in any of them can lead to these kinds of problems. And that's without saying that voltage delivery across a core is an exquisitely analog problem, with voltage fluctuations that might be caused by all sorts of events: instructions being executed, temperature, etc... 27/31
@gabrielesvelto Intel's officially stated reason is that (too) high voltage (and temperature) caused fast degradation of clock trees inside cores. This degradation resulted in a duty cycle shift (square wave no longer square?), which caused general instability. If they use both posedge and negedge as triggers, then change in duty cycle will definitely violate timing.
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@gabrielesvelto Thank you for this detailed and specific explanation. Chris Hobbs discusses the relative unreliability of popular modern CPUs in "Embedded Systems Development for Safety-Critical Systems" but not to this depth.
I don't do embedded work but I do safety-related software QA. Our process has three types of test - acceptance tests which determine fitness-for-use, installation tests to ensure the system is in proper working order, and in-service tests which are sort of a mystery. There's no real guidance on what an in-service test is or how it differs from an installation test. Those are typically run when the operating system is updated or there are similar changes to support software. Given the issue of CPU degradation, I wonder if it makes sense to periodically run in-service tests or somehow detect CPU degradation (that's probably something that should be owned by the infrastructure people vs the application people).
I've mainly thought of CPU failures as design or manufacturing defects, not in terms of "wear" so this has me questioning the assumptions our testing is based on.
@arclight timing degradation should not be visible outside of the highest-spec desktop CPUs which are really pushing the envelope even when they're new. Embedded systems and even mid-range desktop CPUs will never fail because of it. What might become visible is increased power consumption over time though.
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@arclight timing degradation should not be visible outside of the highest-spec desktop CPUs which are really pushing the envelope even when they're new. Embedded systems and even mid-range desktop CPUs will never fail because of it. What might become visible is increased power consumption over time though.
@arclight on the other hand watch out for memory errors. Those can crop up much sooner than CPU problems due to circuit degradation: https://fosstodon.org/@gabrielesvelto/112407741329145666
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In the early days of personal computing CPU bugs were so rare as to be newsworthy. The infamous Pentium FDIV bug is remembered by many, and even earlier CPUs had their own issues (the 6502 comes to mind). Nowadays they've become so common that I encounter them routinely while triaging crash reports sent from Firefox users. Given the nature of CPUs you might wonder how these bugs arise, how they manifest and what can and can't be done about them. 🧵 1/31
@gabrielesvelto there was also no meaningful computer security nor much need for it in the days of 6502. it's much different when most computers are now connected to the internet and can be infected with malware within seconds of connecting.
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@gabrielesvelto but UEFI is already quite complex, it has to find block devices, read their partition tables, read FAT file systems, read directories and files, load data in memory and transfer execution. Wouldn't a patch after all that not be too late?
@mdione yes, it's very complex, but motherboard firmware has a mechanism to load the new microcode right as the CPU is bootstrapped. That is even before the CPU is capable of accessing DRAM. All the rest of the UEFI machinery runs after that. Note that this early bootstrap mechanisms usually involves a separate bootstrap CPU, usually an embedded microcontroller whose task is to get the main x86 core up and running.
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