linux-kernel - Re: [PATCH v2 bpf-next 11/13] bpf: Add bitwise atomic instructions

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Date:   Mon, 30 Nov 2020 09:20:34 -0800
From:   Yonghong Song <yhs@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Brendan Jackman <jackmanb@...gle.com>, <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        KP Singh <kpsingh@...omium.org>,
        Florent Revest <revest@...omium.org>,
        <linux-kernel@...r.kernel.org>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v2 bpf-next 11/13] bpf: Add bitwise atomic instructions



On 11/28/20 5:36 PM, Alexei Starovoitov wrote:
> On Fri, Nov 27, 2020 at 09:39:10PM -0800, Yonghong Song wrote:
>>
>>
>> On 11/27/20 9:57 AM, Brendan Jackman wrote:
>>> This adds instructions for
>>>
>>> atomic[64]_[fetch_]and
>>> atomic[64]_[fetch_]or
>>> atomic[64]_[fetch_]xor
>>>
>>> All these operations are isomorphic enough to implement with the same
>>> verifier, interpreter, and x86 JIT code, hence being a single commit.
>>>
>>> The main interesting thing here is that x86 doesn't directly support
>>> the fetch_ version these operations, so we need to generate a CMPXCHG
>>> loop in the JIT. This requires the use of two temporary registers,
>>> IIUC it's safe to use BPF_REG_AX and x86's AUX_REG for this purpose.
>>
>> similar to previous xsub (atomic[64]_sub), should we implement
>> xand, xor, xxor in llvm?
> 
> yes. please. Unlike atomic_fetch_sub that can be handled by llvm.
> atomic_fetch_or/xor/and has to be seen as separate instructions
> because JITs will translate them as loop.

Okay, will try to implement xsub, xand, xor and xxor in llvm.