Received-SPF: pass (google.com: domain of linux-kernel+bounces-210567-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Date: Tue, 11 Jun 2024 13:15:43 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC: Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [PATCH] x86: add 'runtime constant' infrastructure
User-Agent: K-9 Mail for Android
In-Reply-To: <8eb5960f-17f9-4d94-9b52-dea8b475e9dc@zytor.com>
References: <20240608193504.429644-2-torvalds@linux-foundation.org> <20240610104352.GT8774@noisy.programming.kicks-ass.net> <f967d835-d26e-47af-af35-c3c79746f7d9@rasmusvillemoes.dk> <8eb5960f-17f9-4d94-9b52-dea8b475e9dc@zytor.com>
Message-ID: <BFD0AF77-C95E-4B8B-B475-DCBD808CA5C0@zytor.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain;
 charset=utf-8
Content-Transfer-Encoding: quoted-printable

On June 11, 2024 12:43:02 PM PDT, "H=2E Peter Anvin" <hpa@zytor=2Ecom> wrot=
e:
>On 6/10/24 06:38, Rasmus Villemoes wrote:
>> On 10/06/2024 12=2E43, Peter Zijlstra wrote:
>>> On Sat, Jun 08, 2024 at 12:35:05PM -0700, Linus Torvalds wrote:
>>=20
>>>> Comments?
>>>=20
>>> It obviously has the constraint of never running the code before the
>>> corresponding runtime_const_init() has been done, otherwise things wil=
l
>>> go sideways in a hurry, but this also makes the whole thing a *lot*
>>> simpler=2E
>>>=20
>>> The only thing I'm not sure about is it having a section per symbol,
>>> given how jump_label and static_call took off, this might not be
>>> scalable=2E
>>>=20
>>> Yes, the approach is super simple and straight forward, but imagine
>>> there being like a 100 symbols soon :/
>>>=20
>>> The below hackery -- it very much needs cleanup and is only compiled o=
n
>>> x86_64 and does not support modules, boots for me=2E
>>=20
>> As can be seen in my other reply, yes, I'm also worried about the
>> scalability and would like to see this applied to more stuff=2E
>>=20
>> But if we do this, surely that's what scripts/sorttable is for, right?
>>=20
>> Alternatively, if we just keep emitting to per-symbol
>> __runtime_const_##sym sections but collect them in one __runtime_const,
>> just using __runtime_const { *(SORT_BY_NAME(__runtime_const_*)) } in th=
e
>> linker script should already be enough to allow that binary search to
>> work (with whatever : AT(ADDR() =2E=2E=2E ) magic is also required), wi=
th no
>> post-processing at build or runtime required=2E
>>=20
>
>As far as one section per symbol, this is *exactly* what the linker table=
 infrastructure was intended to make clean and scalable=2E
>
>I think rejecting it was a big mistake=2E It is really a very useful gene=
ral piece of infrastructure, and IMNSHO the whole notion of "oh, we won't e=
ver need that many such tables" is just plain wrong (as evidenced here=2E)
>
>Either way, the problem isn't that hard; you end up doing something like:
>
>struct runtime_const {
>	unsigned int size;
>	reladdr_t entries[0];
>};
>
>#define DECLARE_RUNTIME_CONST(sym,type) \
>extern struct runtime_const sym;\
>asm("=2Epushsection \"runtime_const_" #sym "=2EStart\",\"a\"\n\t"
>    "=2Eglobl " #sym "\n"
>    #sym ": =2Eint 2f - 1f\n\t"
>    "1:\n"
>    "=2Epopsection\n\t"
>    "=2Epushsection \"runtime_const_" #sym "=2E_end\",\"a\"\n\t"
>    "2:\n"
>    "=2Epopsection\n\t");
>
>=2E=2E=2E and add a common suffix, say, "=2Eentry", for the entry section=
 names=2E Then SORT_BY_NAME() will handle the rest=2E
>
>	-hpa
>

Ok, the section naming is obviously bogus, but=2E=2E=2E

I just had an idea how to clearly make this type-safe as a benefit=2E I'm =
at a school event right now but I'll hack up a demo as soon as I get home=
=2E