Notes from the Intelpocalypse

Posted Jan 4, 2018 22:57 UTC (Thu) by roc (subscriber, #30627)
In reply to: Notes from the Intelpocalypse by pizza
Parent article: Notes from the Intelpocalypse

This is completely right.

Also, Itanium would not have been immune to Spectre. Itanium included speculative load operations, and in the "Spectre variant 1" attack, the compiler might well have hoisted the problematic loads above the bounds check precisely to get the performance benefit that an out-of-order CPU gets by speculatively executing those loads.

Notes from the Intelpocalypse

Posted Jan 5, 2018 6:59 UTC (Fri) by epa (subscriber, #39769) [Link] (3 responses)

Right - but on Itanium it would be more straightforward to fix, since you could set a compiler flag to just remove speculative load instructions from the kernel (as a quick fix), adding them back where they are proven safe. Indeed, the compiler could be taught not to speculatively lift loads outside bounds checks.

In user space, I imagine that the explicit speculative load instruction used on Itanium does do all the same memory access checking as an ordinary non-speculative load, so it can't be used to snoop in the same way as the hidden speculative execution on x86_64.

Notes from the Intelpocalypse

Posted Jan 5, 2018 10:12 UTC (Fri) by ortalo (guest, #4654) [Link]

Well, maybe I am a somehow disingenuous, admittedly back then the hardware-based solutions looked better, but I have to question everything, including the fact that the most prominent hardware vendor of that time really did try to favor software development tools rather than its own silicon-oriented intellectual property, don't you think?
Anyway, I would love to be proven wrong and see some of this past research resurrect into a nice powerfull-enough deterministic processor and the associated innovative software development environment for current and near-future critical systems. In my opinion, it is the right time now and many would certainly consider helping it (in good faith I assure you ;-).

Notes from the Intelpocalypse

Posted Jan 5, 2018 11:16 UTC (Fri) by roc (subscriber, #30627) [Link] (1 responses)

Your second paragraph seems to be talking about Meltdown, but Spectre 1 is still a problem for user-space applications. It is probable that Meltdown wouldn't have worked on Itanium.

FWIW in C I don't think it's easy to tell what is a bounds check and which loads are guarded by which checks.

I agree that it would be a bit easier to fix these specific issues in Itanium. I don't think that makes this a "Itanium should have won!" moment.

Notes from the Intelpocalypse

Posted Jan 7, 2018 16:02 UTC (Sun) by mtaht (subscriber, #11087) [Link]

The discussions over at comp.arch have been quite informative,(https://groups.google.com/forum/#!forum/comp.arch)

And it does look like the mill was invulnerable by design to spectre/meltdown. They did find and fix a bug where the compiler could lift a memory access ahead of its guard, but near as I can tell that would have caused a segfault rather than a permissions violation.