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Re: [Bpf] ISA: BPF_MSH and deprecated packet access instructions

Alexei Starovoitov <alexei.starovoitov@gmail.com> Mon, 29 January 2024 00:00 UTC

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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date: Sun, 28 Jan 2024 15:59:48 -0800
Message-ID: <CAADnVQK8JegsSxgbQbO=DR71cRgkvN-y9LH_ZQYxmj1a-hCz5g@mail.gmail.com>
To: Dave Thaler <dthaler1968@googlemail.com>
Cc: "Jose E. Marchesi" <jose.marchesi@oracle.com>, Yonghong Song <yonghong.song@linux.dev>, bpf@ietf.org, bpf <bpf@vger.kernel.org>
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Subject: Re: [Bpf] ISA: BPF_MSH and deprecated packet access instructions
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On Sat, Jan 27, 2024 at 10:59 PM <dthaler1968@googlemail.com> wrote:
>
> I asked:
> > >> What about DW and LDX variants of BPF_IND and BPF_ABS?
>
> Jose E. Marchesi <jose.marchesi@oracle.com> wrote:
> > These we support:
> >
> >   /* Absolute load instructions, designed to be used in socket filters.
> */
> >   {BPF_INSN_LDABSB, "ldabsb%W%i32", "r0 = * ( u8 * ) skb [ %i32 ]",
> >    BPF_V1, BPF_CODE, BPF_CLASS_LD|BPF_SIZE_B|BPF_MODE_ABS},
> >   {BPF_INSN_LDABSH, "ldabsh%W%i32", "r0 = * ( u16 * ) skb [ %i32 ]",
> >    BPF_V1, BPF_CODE, BPF_CLASS_LD|BPF_SIZE_H|BPF_MODE_ABS},
> >   {BPF_INSN_LDABSW, "ldabsw%W%i32", "r0 = * ( u32 * ) skb [ %i32 ]",
> >    BPF_V1, BPF_CODE, BPF_CLASS_LD|BPF_SIZE_W|BPF_MODE_ABS},
> >   {BPF_INSN_LDABSDW, "ldabsdw%W%i32", "r0 = * ( u64 * ) skb [ %i32 ]",
> >    BPF_V1, BPF_CODE, BPF_CLASS_LD|BPF_SIZE_DW|BPF_MODE_ABS},
> >
> >   /* Generic load instructions (to register.)  */
> >   {BPF_INSN_LDXB, "ldxb%W%dr , [ %sr %o16 ]", "%dr = * ( u8 * ) ( %sr %o16
> )",
> >    BPF_V1, BPF_CODE, BPF_CLASS_LDX|BPF_SIZE_B|BPF_MODE_MEM},
> >   {BPF_INSN_LDXH, "ldxh%W%dr , [ %sr %o16 ]", "%dr = * ( u16 * ) ( %sr
> %o16
> > )",
> >    BPF_V1, BPF_CODE, BPF_CLASS_LDX|BPF_SIZE_H|BPF_MODE_MEM},
> >   {BPF_INSN_LDXW, "ldxw%W%dr , [ %sr %o16 ]", "%dr = * ( u32 * ) ( %sr
> %o16
> > )",
> >    BPF_V1, BPF_CODE, BPF_CLASS_LDX|BPF_SIZE_W|BPF_MODE_MEM},
> >   {BPF_INSN_LDXDW, "ldxdw%W%dr , [ %sr %o16 ]","%dr = * ( u64 * ) ( %sr
> > %o16 )",
> >    BPF_V1, BPF_CODE, BPF_CLASS_LDX|BPF_SIZE_DW|BPF_MODE_MEM},
>
> Yonghong Song <yonghong.song@linux.dev> wrote:
> > I don't know how to do proper wording in the standard. But DW and LDX
> > variants of BPF_IND/BPF_ABS are not supported by verifier for now and they
> > are considered illegal insns.
>
> Although the Linux verifier doesn't support them, the fact that gcc does
> support
> them tells me that it's probably safest to list the DW and LDX variants as
> deprecated as well, which is what the draft already did in the appendix so
> that's good (nothing to change there, I think).

DW never existed in classic bpf, so abs/ind never had DW flavor.
If some assembler/compiler decided to "support" them it's on them.
The standard must not list such things as deprecated. They never
existed. So nothing is deprecated.
Same with MSH. BPF_LDX | BPF_MSH | BPF_B is the only insn ever existed.
It's a legacy insn. Just like abs/ind.

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