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January 22, 2018 08:00 am

Linus Torvalds Calls Intel Patches 'Complete and Utter Garbage'

An anonymous reader writes:On the Linux Kernel Mailing List, Linus Torvalds ended up responding to a long-time kernel developer (and former Intel engineer) who'd been describing a new microcode feature addressing Indirect Branch Restricted Speculation "where a future CPU will advertise 'I am able to be not broken' and then you have to set the IBRS bit once at boot time to *ask* it not to be broken." Linus calls it "very much part of the whole 'this is complete garbage' issue. The whole IBRS_ALL feature to me very clearly says 'Intel is not serious about this, we'll have a ugly hack that will be so expensive that we don't want to enable it by default, because that would look bad in benchmarks'. So instead they try to push the garbage down to us. And they are doing it entirely wrong, even from a technical standpoint. I'm sure there is some lawyer there who says 'we'll have to go through motions to protect against a lawsuit'. But legal reasons do not make for good technology, or good patches that I should apply." Later Linus says forcefully that these "complete and utter garbage" patches are being pushed by someone "for unclear reasons" -- and adds another criticism.The whole point of having cpuid and flags from the microarchitecture is that we can use those to make decisions. But since we already know that the IBRS overhead is huge on existing hardware, all those hardware capability bits are just complete and utter garbage. Nobody sane will use them, since the cost is too damn high. So you end up having to look at "which CPU stepping is this" anyway. I think we need something better than this garbage.

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