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Date: Wed, 29 May 2024 17:15:31 +0200
From: Borislav Petkov <bp@alien8.de>
To: Andrew Cooper <andrew.cooper3@citrix.com>,
	Nikolay Borisov <nik.borisov@suse.com>
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Subject: Re: [PATCHv11 05/19] x86/relocate_kernel: Use named labels for less
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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].
>=20
> 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.
>=20
> In those days, the prefetch queue was not coherent with main memory, an=
d
> jumps (being a discontinuity in the instruction stream) simply flushed
> the prefetch queue.
>=20
> 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.=C2=A0 (Otherwise known as the way to distin=
guish
> the 8086 from the 8088 because the latter only had a 4 byte prefetch qu=
eue.)

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

> Anyway.=C2=A0 It's how you used to spell "serialising operation" before=
 that
> term ever entered the architecture.=C2=A0 Linux still supports CPUs pri=
or to
> the Pentium, so still needs to care about prefetch queues in the 486.
>=20
> 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 fro=
m
> 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
>=20
> 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!

--=20
Regards/Gruss,
    Boris.

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