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From: Yonghong Song <yonghong.song@linux.dev>
To: dthaler1968@googlemail.com
Cc: bpf@ietf.org, bpf@vger.kernel.org
Subject: Re: Sign extension ISA question
Date: Tue, 16 Jan 2024 19:48:44 -0800	[thread overview]
Message-ID: <4dfb0d6a-aa48-4d96-82f0-09a960b1012f@linux.dev> (raw)
In-Reply-To: <095f01da48e8$611687d0$23439770$@gmail.com>

On 1/16/24 5:56 PM, dthaler1968@googlemail.com wrote:
> Yonghong Song <yonghong.song@linux.dev> wrote:
>>> Is there any semantic difference between the following two instructions?
>>>
>>> {.opcode = BPF_ALU64 | BPF_MOV | BPF_K, .offset = 0, .imm = -1}
>> This is supported. Sign extension of -1 will be put into ALU64 reg.
>>
>>> {.opcode = BPF_ALU64 | BPF_MOVSX | BPF_K, .offset = 32, .imm = -1}
>> This is not supported. BPF_MOVSX only supports register extension.
>> We should make it clear in the doc.
> Is that limitation a Linux-specific implementation statement? (i.e., put into
> linux-notes.txt)
>
> Or that the meaning is undefined for all runtimes and could be used
> for some other purpose in the future?  (i.e., put into instruction-set.rst)
>
> For now I'll interpret it as the latter.

You are right. The
   {.opcode = BPF_ALU64 | BPF_MOVSX | BPF_K, .offset = 32, .imm = -1}
is not supported by bpf ISA. Currently, it will be an illegal encoding
from kernel perspective.


WARNING: multiple messages have this Message-ID (diff)
From: Yonghong Song <yonghong.song@linux.dev>
To: dthaler1968@googlemail.com
Cc: bpf@ietf.org, bpf@vger.kernel.org
Subject: Re: [Bpf] Sign extension ISA question
Date: Tue, 16 Jan 2024 19:48:44 -0800	[thread overview]
Message-ID: <4dfb0d6a-aa48-4d96-82f0-09a960b1012f@linux.dev> (raw)
Message-ID: <20240117034844.FKGHK5gRhW2hNPuMu9rYrK2q57pDZCRqs3Nr2IuhcmE@z> (raw)
In-Reply-To: <095f01da48e8$611687d0$23439770$@gmail.com>

On 1/16/24 5:56 PM, dthaler1968@googlemail.com wrote:
> Yonghong Song <yonghong.song@linux.dev> wrote:
>>> Is there any semantic difference between the following two instructions?
>>>
>>> {.opcode = BPF_ALU64 | BPF_MOV | BPF_K, .offset = 0, .imm = -1}
>> This is supported. Sign extension of -1 will be put into ALU64 reg.
>>
>>> {.opcode = BPF_ALU64 | BPF_MOVSX | BPF_K, .offset = 32, .imm = -1}
>> This is not supported. BPF_MOVSX only supports register extension.
>> We should make it clear in the doc.
> Is that limitation a Linux-specific implementation statement? (i.e., put into
> linux-notes.txt)
>
> Or that the meaning is undefined for all runtimes and could be used
> for some other purpose in the future?  (i.e., put into instruction-set.rst)
>
> For now I'll interpret it as the latter.

You are right. The
   {.opcode = BPF_ALU64 | BPF_MOVSX | BPF_K, .offset = 32, .imm = -1}
is not supported by bpf ISA. Currently, it will be an illegal encoding
from kernel perspective.

-- 
Bpf mailing list
Bpf@ietf.org
https://www.ietf.org/mailman/listinfo/bpf

  reply	other threads:[~2024-01-17  3:48 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-01-16 20:38 [Bpf] Sign extension ISA question dthaler1968=40googlemail.com
2024-01-16 20:55 ` dthaler1968
2024-01-16 20:55   ` [Bpf] " dthaler1968=40googlemail.com
2024-01-16 22:34   ` Yonghong Song
2024-01-16 22:34     ` [Bpf] " Yonghong Song
2024-01-17  1:56     ` dthaler1968
2024-01-17  1:56       ` [Bpf] " dthaler1968=40googlemail.com
2024-01-17  3:48       ` Yonghong Song [this message]
2024-01-17  3:48         ` Yonghong Song
2024-01-24  2:07         ` Jump instructions clarification dthaler1968
2024-01-24  2:07           ` [Bpf] " dthaler1968=40googlemail.com
2024-01-24 19:33           ` Yonghong Song
2024-01-24 19:33             ` [Bpf] " Yonghong Song
2024-01-26  1:12             ` 64-bit immediate " dthaler1968
2024-01-26  1:12               ` [Bpf] " dthaler1968=40googlemail.com
2024-01-26  5:34               ` Yonghong Song
2024-01-26  5:34                 ` [Bpf] " Yonghong Song
2024-01-26 22:27                 ` dthaler1968
2024-01-26 22:27                   ` [Bpf] " dthaler1968=40googlemail.com
2024-01-27  3:41                   ` Yonghong Song
2024-01-27  3:41                     ` [Bpf] " Yonghong Song
2024-01-27  6:56                     ` dthaler1968
2024-01-27  6:56                       ` [Bpf] " dthaler1968=40googlemail.com

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