From: Eduard Zingerman <eddyz87@gmail.com>
To: Vineet Gupta <vineet.gupta@linux.dev>,
ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
memxor@gmail.com
Cc: martin.lau@linux.dev, song@kernel.org, yonghong.song@linux.dev,
jolsa@kernel.org, emil@etsalapatis.com, ihor.solodrai@linux.dev,
john.fastabend@gmail.com, shuah@kernel.org, bpf@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org
Subject: Re: [PATCH bpf-next v3 11/12] [do not merge] bpf: Track low-32 scalar equality across narrowing stack spills
Date: Fri, 30 Oct 2026 11:33:04 -0700 [thread overview]
Message-ID: <727459335119599b851b7a1ed72dbe6220e14571.camel@gmail.com> (raw)
In-Reply-To: <20261020161907.2115379-12-vineet.gupta@linux.dev>
On Tue, 2026-10-20 at 09:19 -0700, Vineet Gupta wrote:
> A spill narrower than its source stores the source's low 32 bits, so the
> slot is their zero-extension -- the mov relation, in the store direction:
>
> r6 = ... /* full 64-bit unknown */
> *(u32 *)(r10 - 8) = r6; /* slot holds r6's low 32 bits */
> if w6 != 0 goto ... /* not taken: r6's low 32 bits are 0 */
> r2 = *(u32 *)(r10 - 8);
> if r2 == 0 goto ... /* not deduced today */
>
> A store never sign-extends, so BPF_REG_LINK_ZEXT is the only kind that
> arises. save_register_state() copies the source whole, including bounds
> for bits the store did not write, so the slot must be truncated before
> the kind goes on it -- otherwise the first sync_linked_regs() intersects
> the correctly rebuilt low half against those stale bounds and empties
> the slot.
>
> Only one order pays. sync_linked_regs() walks spilled slots, so narrowing
> while the value is still on the stack narrows the slot and the fill copies
> an already narrowed value. Filling first does not benefit: the fill is a
> mov, and a mov drops a zero-extending link.
>
> Posted, not proposed. This is the direction asked for, implemented and
> measured, and the measurement says do not take it. Against the ten patches
> before it, on 441 cilium/katran/fb/sched_ext objects, 1467 programs:
>
> verdicts no change at all
> total_insns 102 changed, 63 worse, +84,450 (+0.34%)
> +59,616 (+57.2%) 104,160 -> 163,776 test_file_open
> +19,784 (+89.7%) 22,053 -> 41,837 rusty_enqueue
> +2,688 (+40.6%) 6,626 -> 9,314 layered_enqueue
> total_states 64 changed, 63 worse
I'd still prefer to keep things symmetric.
Instruction budget should be less of a concert for 7.5,
as far as I understand.
...
next prev parent reply other threads:[~2026-10-30 18:33 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-20 16:18 [PATCH bpf-next v3 00/12] bpf: track scalar equality across the low 32 bits Vineet Gupta
2026-10-20 16:18 ` [PATCH bpf-next v3 01/12] bpf: Keep the 32-bit range across a sign extension that cannot change it Vineet Gupta
2026-10-29 22:30 ` Eduard Zingerman
2026-10-20 16:18 ` [PATCH bpf-next v3 02/12] selftests/bpf: Cover sign extensions that cannot change the range Vineet Gupta
2026-10-20 16:18 ` [PATCH bpf-next v3 03/12] bpf: Prepare linked scalars for relations narrower than 64 bits Vineet Gupta
2026-10-20 17:08 ` bot+bpf-ci
2026-10-26 22:52 ` Vineet Gupta
2026-10-29 22:51 ` Eduard Zingerman
2026-10-20 16:18 ` [PATCH bpf-next v3 04/12] bpf: Track low-32 scalar equality across zero-extending movs Vineet Gupta
2026-10-20 17:09 ` bot+bpf-ci
2026-10-26 20:30 ` Vineet Gupta
2026-10-29 23:59 ` Eduard Zingerman
2026-10-20 16:19 ` [PATCH bpf-next v3 05/12] selftests/bpf: Cover the low-32 link for " Vineet Gupta
2026-10-20 16:53 ` bot+bpf-ci
2026-10-26 18:36 ` Vineet Gupta
2026-10-20 16:19 ` [PATCH bpf-next v3 06/12] bpf: Track low-32 scalar equality across sign-extending movs Vineet Gupta
2026-10-30 17:52 ` Eduard Zingerman
2026-10-20 16:19 ` [PATCH bpf-next v3 07/12] selftests/bpf: Cover the low-32 link for " Vineet Gupta
2026-10-20 16:19 ` [PATCH bpf-next v3 08/12] bpf: Recover the low 32 bits on the reverse link direction Vineet Gupta
2026-10-20 16:53 ` bot+bpf-ci
2026-10-26 19:57 ` Vineet Gupta
2026-10-20 16:19 ` [PATCH bpf-next v3 09/12] bpf: Track low-32 scalar equality across narrowing stack fills Vineet Gupta
2026-10-30 18:27 ` Eduard Zingerman
2026-10-20 16:19 ` [PATCH bpf-next v3 10/12] selftests/bpf: Cover the low-32 link for " Vineet Gupta
2026-10-20 16:52 ` bot+bpf-ci
2026-10-26 20:21 ` Vineet Gupta
2026-10-20 16:19 ` [PATCH bpf-next v3 11/12] [do not merge] bpf: Track low-32 scalar equality across narrowing stack spills Vineet Gupta
2026-10-30 18:33 ` Eduard Zingerman [this message]
2026-10-20 16:19 ` [PATCH bpf-next v3 12/12] [do not merge] selftests/bpf: Cover the low-32 link for " Vineet Gupta
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