The mysterious case of the Linux Page Table Isolation patches

Re:The mysterious case of the Linux Page Table Isolation pat... by userbinator on Monday January 01, 2018 @12:42PM (#16047026)

Radioactive decays and cosmic particles flipping bits give an upper bound for reliability well below 99.9999999999%

If it works 99.9999999999%, then it has a failure rate of 0.0000000001%, or 1E-12. Considering that a modern CPU executes approximately 1E9 operations per second, and that regular HDDs have a worse-case BER of 1 in 1E14 bits, 1E-12 is actually rather horrible and the actual error rate of computer hardware is much better than that.

Imagine if a CPU calculated 1+1=3 every 1E12 instructions. At current clock rates, that's a fraction of an hour. Computers simply would not work if CPUs had such an error rate.

I picked the 1E-12 number arbitrarily, but it's quite illustrative of the reliability computers are expected to have, despite their flaws.

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Re:The mysterious case of the Linux Page Table Isolation pat... by avian on Monday January 01, 2018 @01:26PM (#16047241)

I edited my reply to remove the actual number. The argument still stands that you cannot expect 100% reliability.

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Re:The mysterious case of the Linux Page Table Isolation pat... by blattimwind on Monday January 01, 2018 @02:50PM (#16047676)

Generally speaking, a piece of JavaScript in some 0x0 pixel iframe in a tab you're not even looking at can't summon cosmic particles to manipulate your computer's main memory. Rowhammer can.

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Re:The mysterious case of the Linux Page Table Isolation pat... by monochromatic on Monday January 01, 2018 @02:02PM (#16047440)

A small probability of random errors can be lived with. Errors that can be triggered and exploited are unacceptable.

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Re:The mysterious case of the Linux Page Table Isolation pat... by brians on Monday January 01, 2018 @04:19PM (#16048111)

No, but it’s straightforward engineering to prevent many of these problems. Error-correcting codes have been well understood for most of a century. Yes, it costs in performance and in money. For life-safety applications, it shouldn’t be optional.

Amazon is hosting life-safety applications in EC2. Commodity x86 hardware is grossly negligent for that environment.

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Re:The mysterious case of the Linux Page Table Isolation pat... by jchw on Monday January 01, 2018 @07:55PM (#16049020)

ECC as it is today may not be sufficient. It's mostly useful against single bit flips. Probably better than nothing, though...

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Re:The mysterious case of the Linux Page Table Isolation pat... by brians on Monday January 01, 2018 @10:43PM (#16049709)

ECC alone is absolutely insufficient. But ECC can be part of a system design that includes active monitoring and response. I’d expect that system design to also include measurement of ECC events under ordinary conditions, regular re-measurement, and funding for an analysis of the changes and explanation of the difference—just like you’d find in safety engineering in a coal plant, an MRI machine, any sort of engineering that has a professional scientist or engineer on site supervising all operations.

Of course, you’ll also find a tendency there towards specified hardware. They bend or break it to use COTS x86 machines, but—as I think I heard from a comment here last week—nearly nobody ever specified wanting AMT in the initial design, so it’s pretty weird that we’re all buying and deploying it.

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Re:The mysterious case of the Linux Page Table Isolation pat... by emilfihlman on Wednesday January 03, 2018 @12:02PM (#16062688)

This is not how probabilities work but eh.

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