Re: [PATCHv11 05/19] x86/relocate_kernel: Use named labels for less confusion

[Borislav Petkov]

On Wed, May 29, 2024 at 01:33:35PM +0100, Andrew Cooper wrote:
> Seems I've gained a reputation...

Yes you have. You have this weird interest in very deep uarch details
that I can't share. Not at that detail. :-P

> jmp 1f dates back to ye olde 8086, which started the whole trend of the
> instruction pointer just being a figment of the ISA's imagination[1].
> 
> Hardware maintains the pointer to the next byte to fetch (the prefetch
> queue was up to 6 bytes), and there was a micro-op to subtract the
> current length of the prefetch queue from the accumulator.
> 
> In those days, the prefetch queue was not coherent with main memory, and
> jumps (being a discontinuity in the instruction stream) simply flushed
> the prefetch queue.
> 
> This was necessary after modifying executable code, because otherwise
> you could end up executing stale bytes from the prefetch queue and then
> non-stale bytes thereafter.  (Otherwise known as the way to distinguish
> the 8086 from the 8088 because the latter only had a 4 byte prefetch queue.)

Thanks - that certainly wakes up a long-asleep neuron in the back of my
mind...

> Anyway.  It's how you used to spell "serialising operation" before that
> term ever entered the architecture.  Linux still supports CPUs prior to
> the Pentium, so still needs to care about prefetch queues in the 486.
> 
> However, this example appears to be in 64bit code and following a write
> to CR4 which will be fully serialising, so it's probably copy&paste from
> 32bit code where it would be necessary in principle.

Yap, fully agreed. We could try to remove it and see what complains.

Nikolay, wanna do a patch which properly explains the situation?

> https://www.righto.com/2023/01/inside-8086-processors-instruction.html#fn:pc
> 
> In fact, anyone who hasn't should read the entire series on the 8086,
> https://www.righto.com/p/index.html

Oh yeah, already bookmarked.

Thanks Andy!

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette