linux-kernel - Re: [RFC PATCH 3/3] x86/boot: Use alternatives based selector for 5-level paging constants

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Message-ID: <CAHk-=wiz5oXq2f_87hHViN2TZQO9VHpaWb5fWWGJbUWQw1ChVw@mail.gmail.com>
Date: Tue, 6 May 2025 10:43:51 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, x86@...nel.org, 
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC PATCH 3/3] x86/boot: Use alternatives based selector for
 5-level paging constants

On Tue, 6 May 2025 at 10:26, Ard Biesheuvel <ardb@...nel.org> wrote:
>
> In the light of the above, care to comment on the previous approach?
>
> https://lore.kernel.org/all/20250504095230.2932860-28-ardb+git@google.com/

I have to say, I find that one much more palatable.

That said, I still think it would be better to get rid of the early
case *entirely*.

 So it would be lovely to have a subsequent patch that just makes the
"before fixup" case result in an UD2 instead of "read cr4 and check
the LA57 bit" and then fix the fallout.

I think that fallout could be handled by having it be an exception
that prints out a warning, and then jumps to the right target.
Anything else would be very painful (as in "oh, the machion doesn't
boot, because things go wrong during early boot").

But I think that first version of yours is simpler and more
straightforward than the later alternatives, and it does get rid of
that nasty USE_EARLY_PGTABLE_L5 thing.

I just think that in a perfect world we could then do more cleanups on
top of that.

              Linus