[adrian_b]

They had to disable AVX-512 only because Microsoft was too lazy to rewrite their thread scheduler to handle heterogeneous CPU cores.

The Intel-AMD x86-64 architecture is full of horrible things, starting with the System Management Mode added in 1990, which have been added by Intel only because every time Microsoft has refused to update Windows, expecting that the hardware vendors must do the work instead of Microsoft for enabling Windows to continue to work on newer hardware, even when that causes various disadvantages for the customers.

Moreover, even if Intel had not said that Alder Lake will support AVX-512, they also had not said that the P-cores of Alder Lake will not support AVX-512.

Therefore everybody had expected that Intel will continue to provide backward compatibility, as always before that, so the P-cores of Alder Lake will continue to support any instruction subset that had been supported by Rocket Lake and Tiger Lake and Ice Lake and Cannon Lake.

The failure to be compatible with their previous products has been a surprise for everybody.

[p_l]

Windows can work without SMM, especially NT - the problem is that SMM was created for a world where majority used DOS and the idea of using OS services instead of every possibly quirk of IBM PC was anathema to developers.

Thus, SMM, because there was no other way to hook power management on a 386 laptop running " normal" DOS

[cesarb]

> Thus, SMM, because there was no other way to hook power management on a 386 laptop running " normal" DOS

In theory, there was: you could have a separate microcontroller, accessed through some of the I/O ports, doing the power management; it's mostly how it's done nowadays, with the EC (Embedded Controller) on laptops (and nowadays there's also the PSP or ME, which is a separate processor core doing startup and power management for the main CPU cores). But back then, it would also be more expensive (a whole other chip) than simply adding an extra mode to the single CPU core (multiple cores back then usually required multiple CPU chips).

[p_l]

The problem is reliably interrupting the CPU in a way that didn't require extra OS support. SMM provided such trigger, and in fact is generally used as part of the scheme with EC cooperating.

[kccqzy]

If Windows could work without SMM, is there a historical reason why SMM mode didn't just die and become disused after Windows becomes popular and nobody uses DOS any more? There are plenty of features in x86 that are disused.

[p_l]

The feature turned out too useful for all sorts of things, including dealing with the fact that before NT loaded itself you still had to emulate being an IBM PC including the fiction of booting from cassette tape or jumping to ROM BASIC.

Also, it's been cheaper to implement various features through small piece of code instead of adding a separate MCU to handle them, including prosaic things like handling NVRAM storage for variables (instead of interacting with external MCU or having separate NVRAM, you end up with SMM code being "trusted" to update the homogenous flash chip that contains both NVRAM and boot code)

[Symmetry]

I don't know if I'd call Microsoft lazy. Are there any existing operating systems that allow preemptive scheduling across cores with different ISA subsets? I'd sort of assume Microsoft research has a proof of concept for something like that but putting it into a production OS is a different kettle of fish.

[kccqzy]

> the P-cores of Alder Lake will continue to support any instruction subset that had been supported by Rocket Lake and Tiger Lake and Ice Lake and Cannon Lake

Wait. I thought the article says only Tiger Lake supports the vp2intersect instruction. Is that not true then?

[adrian_b]

Tiger Lake is the only one with vp2intersect, but before Alder Lake there had already been 3 generations of consumer CPUs with AVX-512 support (Cannon Lake in 2018/2019, Ice Lake in 2019/2020 and Tiger Lake + Rocket Lake in 2020/2021).

So it was expected that any future Intel CPUs will remain compatible. Removing an important instruction subset has never happened before in Intel's history.

Only AMD has removed some instructions when passing from a 32-bit ISA to a 64-bit ISA, most of which were obsolete (except that removing interrupt on overflow was bad and it does not simplify greatly a CPU core, since there are many other sources of precise exceptions that must still be supported; the only important effect of removing INTO is that many instructions can be retired earlier than otherwise, which reduces the risk of filling up the retirement queue).