From: "Eelco Chaudron" <echaudro@redhat.com>
To: "Maciej Fijalkowski" <maciej.fijalkowski@intel.com>
Cc: "Lorenzo Bianconi" <lorenzo@kernel.org>,
bpf@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: [PATCH v5 bpf-next 13/14] bpf: add new frame_length field to the XDP ctx
Date: Wed, 16 Dec 2020 15:08:28 +0100 [thread overview]
Message-ID: <54E66B9D-4677-436F-92A1-E70977E869FA@redhat.com> (raw)
In-Reply-To: <20201215180638.GB23785@ranger.igk.intel.com>
On 15 Dec 2020, at 19:06, Maciej Fijalkowski wrote:
> On Tue, Dec 15, 2020 at 02:28:39PM +0100, Eelco Chaudron wrote:
>>
>>
>> On 9 Dec 2020, at 13:07, Eelco Chaudron wrote:
>>
>>> On 9 Dec 2020, at 12:10, Maciej Fijalkowski wrote:
>>
>> <SNIP>
>>
>>>>>>> +
>>>>>>> + ctx_reg = (si->src_reg == si->dst_reg) ? scratch_reg - 1 :
>>>>>>> si->src_reg;
>>>>>>> + while (dst_reg == ctx_reg || scratch_reg == ctx_reg)
>>>>>>> + ctx_reg--;
>>>>>>> +
>>>>>>> + /* Save scratch registers */
>>>>>>> + if (ctx_reg != si->src_reg) {
>>>>>>> + *insn++ = BPF_STX_MEM(BPF_DW, si->src_reg, ctx_reg,
>>>>>>> + offsetof(struct xdp_buff,
>>>>>>> + tmp_reg[1]));
>>>>>>> +
>>>>>>> + *insn++ = BPF_MOV64_REG(ctx_reg, si->src_reg);
>>>>>>> + }
>>>>>>> +
>>>>>>> + *insn++ = BPF_STX_MEM(BPF_DW, ctx_reg, scratch_reg,
>>>>>>> + offsetof(struct xdp_buff, tmp_reg[0]));
>>>>>>
>>>>>> Why don't you push regs to stack, use it and then pop it
>>>>>> back? That way
>>>>>> I
>>>>>> suppose you could avoid polluting xdp_buff with tmp_reg[2].
>>>>>
>>>>> There is no “real” stack in eBPF, only a read-only frame
>>>>> pointer, and as we
>>>>> are replacing a single instruction, we have no info on what we
>>>>> can use as
>>>>> scratch space.
>>>>
>>>> Uhm, what? You use R10 for stack operations. Verifier tracks the
>>>> stack
>>>> depth used by programs and then it is passed down to JIT so that
>>>> native
>>>> asm will create a properly sized stack frame.
>>>>
>>>> From the top of my head I would let know xdp_convert_ctx_access of a
>>>> current stack depth and use it for R10 stores, so your scratch space
>>>> would
>>>> be R10 + (stack depth + 8), R10 + (stack_depth + 16).
>>>
>>> Other instances do exactly the same, i.e. put some scratch registers in
>>> the underlying data structure, so I reused this approach. From the
>>> current information in the callback, I was not able to determine the
>>> current stack_depth. With "real" stack above, I meant having a pop/push
>>> like instruction.
>>>
>>> I do not know the verifier code well enough, but are you suggesting I
>>> can get the current stack_depth from the verifier in the
>>> xdp_convert_ctx_access() callback? If so any pointers?
>>
>> Maciej any feedback on the above, i.e. getting the stack_depth in
>> xdp_convert_ctx_access()?
>
> Sorry. I'll try to get my head around it. If i recall correctly stack
> depth is tracked per subprogram whereas convert_ctx_accesses is iterating
> through *all* insns (so a prog that is not chunked onto subprogs), but
> maybe we could dig up the subprog based on insn idx.
>
> But at first, you mentioned that you took the approach from other
> instances, can you point me to them?
Quick search found the following two (sure there is one more with two regs):
https://elixir.bootlin.com/linux/v5.10.1/source/kernel/bpf/cgroup.c#L1718
https://elixir.bootlin.com/linux/v5.10.1/source/net/core/filter.c#L8977
> I'd also like to hear from Daniel/Alexei/John and others their thoughts.
Please keep me in the loop…
>>
>>>> Problem with that would be the fact that convert_ctx_accesses()
>>>> happens to
>>>> be called after the check_max_stack_depth(), so probably stack_depth
>>>> of a
>>>> prog that has frame_length accesses would have to be adjusted
>>>> earlier.
>>>
>>> Ack, need to learn more on the verifier part…
>>
>> <SNIP>
>>
next prev parent reply other threads:[~2020-12-16 14:10 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-07 16:32 [PATCH v5 bpf-next 00/14] mvneta: introduce XDP multi-buffer support Lorenzo Bianconi
2020-12-07 16:32 ` [PATCH v5 bpf-next 01/14] xdp: introduce mb in xdp_buff/xdp_frame Lorenzo Bianconi
2020-12-07 21:16 ` Alexander Duyck
2020-12-07 23:03 ` Saeed Mahameed
2020-12-08 3:16 ` Alexander Duyck
2020-12-08 6:49 ` Saeed Mahameed
2020-12-08 9:47 ` Jesper Dangaard Brouer
2020-12-07 16:32 ` [PATCH v5 bpf-next 02/14] xdp: initialize xdp_buff mb bit to 0 in all XDP drivers Lorenzo Bianconi
2020-12-07 21:15 ` Alexander Duyck
2020-12-07 21:37 ` Maciej Fijalkowski
2020-12-07 23:20 ` Saeed Mahameed
2020-12-08 10:31 ` Lorenzo Bianconi
2020-12-08 13:29 ` Jesper Dangaard Brouer
2020-12-07 16:32 ` [PATCH v5 bpf-next 03/14] xdp: add xdp_shared_info data structure Lorenzo Bianconi
2020-12-08 0:22 ` Saeed Mahameed
2020-12-08 11:01 ` Lorenzo Bianconi
2020-12-19 14:53 ` Shay Agroskin
2020-12-19 15:30 ` Jamal Hadi Salim
2020-12-21 9:01 ` Jesper Dangaard Brouer
2020-12-21 13:00 ` Jamal Hadi Salim
2020-12-20 17:52 ` Lorenzo Bianconi
2020-12-21 20:55 ` Shay Agroskin
2020-12-07 16:32 ` [PATCH v5 bpf-next 04/14] net: mvneta: update mb bit before passing the xdp buffer to eBPF layer Lorenzo Bianconi
2020-12-07 16:32 ` [PATCH v5 bpf-next 05/14] xdp: add multi-buff support to xdp_return_{buff/frame} Lorenzo Bianconi
2020-12-07 16:32 ` [PATCH v5 bpf-next 06/14] net: mvneta: add multi buffer support to XDP_TX Lorenzo Bianconi
2020-12-19 15:56 ` Shay Agroskin
2020-12-20 18:06 ` Lorenzo Bianconi
2020-12-07 16:32 ` [PATCH v5 bpf-next 07/14] bpf: move user_size out of bpf_test_init Lorenzo Bianconi
2020-12-07 16:32 ` [PATCH v5 bpf-next 08/14] bpf: introduce multibuff support to bpf_prog_test_run_xdp() Lorenzo Bianconi
2020-12-07 16:32 ` [PATCH v5 bpf-next 09/14] bpf: test_run: add xdp_shared_info pointer in bpf_test_finish signature Lorenzo Bianconi
2020-12-07 16:32 ` [PATCH v5 bpf-next 10/14] net: mvneta: enable jumbo frames for XDP Lorenzo Bianconi
2020-12-07 16:32 ` [PATCH v5 bpf-next 11/14] bpf: cpumap: introduce xdp multi-buff support Lorenzo Bianconi
2020-12-19 17:46 ` Shay Agroskin
2020-12-20 17:56 ` Lorenzo Bianconi
2020-12-07 16:32 ` [PATCH v5 bpf-next 12/14] bpf: add multi-buff support to the bpf_xdp_adjust_tail() API Lorenzo Bianconi
2020-12-07 16:32 ` [PATCH v5 bpf-next 13/14] bpf: add new frame_length field to the XDP ctx Lorenzo Bianconi
2020-12-08 22:17 ` Maciej Fijalkowski
2020-12-09 10:35 ` Eelco Chaudron
2020-12-09 11:10 ` Maciej Fijalkowski
2020-12-09 12:07 ` Eelco Chaudron
2020-12-15 13:28 ` Eelco Chaudron
2020-12-15 18:06 ` Maciej Fijalkowski
2020-12-16 14:08 ` Eelco Chaudron [this message]
2021-01-15 16:36 ` Eelco Chaudron
2021-01-18 16:48 ` Maciej Fijalkowski
2021-01-20 13:20 ` Eelco Chaudron
2021-02-01 16:00 ` Eelco Chaudron
2020-12-07 16:32 ` [PATCH v5 bpf-next 14/14] bpf: update xdp_adjust_tail selftest to include multi-buffer Lorenzo Bianconi
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