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Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.13.0 Subject: Re: [PATCH v4 00/25] RSEQ node id and virtual cpu id extensions Content-Language: en-US To: Chris Kennelly References: <20220922105941.237830-1-mathieu.desnoyers@efficios.com> From: Mathieu Desnoyers Cc: Peter Zijlstra , Paul Turner , Peter Oskolkov , linux-kernel , "carlos@redhat.com" , Florian Weimer , "linux-api@vger.kernel.org" In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: YQBPR01CA0080.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:3::16) To YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: YT2PR01MB9175:EE_|YQBPR0101MB5687:EE_ X-MS-Office365-Filtering-Correlation-Id: f7964e4f-d1b3-4123-6300-08da9d69e056 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM;PTR:;CAT:NONE;SFS:(13230022)(39830400003)(34096005)(366004)(396003)(346002)(376002)(136003)(34036004)(451199015)(6666004)(966005)(41300700001)(498600001)(586005)(38610400001)(6486002)(4326008)(66946007)(36756003)(8676002)(54906003)(6916009)(38100700002)(66556008)(6506007)(53546011)(66476007)(2616005)(31696002)(86362001)(26005)(186003)(6512007)(41320700001)(83380400001)(31686004)(44832011)(5660300002)(8936002)(2906002)(43740500002)(45980500001);DIR:OUT;SFP:1101; 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If you have a concrete example of this, I'd be happy to help and perhaps we can improve your usage pattern. > > I think I can avoid this by using %gs to point to the address of the > cpu_id field itself (which I think could be used to select between vCPUs > or not*), but %gs is a global piece of state that all of the libraries > in the program need to cooperate on. I think what we are all looking for here is a scheme that would allow us the fastest per-vcpu data structure accesses possible from userspace. I think we could do something similar to what is done in the Linux kernel for that, but in userspace. Here are some random ideas I have on this topic: We could introduce a new prctl(2) PT_{SET,GET}_GS_MODE on x86-64. This would take as arguments the indexing mode and offset multiplier we want to be applied to the GS segment selector on return to userspace: enum gs_index_mode { GS_INDEX_MODE_MM_VCPU, }; struct prctl_set_gs_mode { enum gs_index_mode index_mode; u64 stride; }; For a memory space which has this gs mode set, the return to userspace code would populate the GS segment selector register with: stride * current->mm_vcpu_id The "stride" would be the virtual address space size allowed for per-vcpu-data. This could be decided by the libc, with a tunable allowing to increase/decrease this size. Another libc tunable could disable populating the GS segment selector altogether (e.g. for compatibility with applications like Wine which AFAIK use it). With this in place, I hope we could then do per-vcpu data access by simply prefixing memory access instructions with a %%gs: segment selector prefix. Thoughts ? Thanks, Mathieu > > Thanks, > Chris > > * TCMalloc is already paying a load+pointer arithmetic to select between > cpu_id versus vcpu_id, so this would actually make things a little bit > faster. > > On Thu, Sep 22, 2022 at 3:21 PM Mathieu Desnoyers > > > wrote: > > Hi Chris, > > Sorry it looks like I forgot to CC you on this series. If you can give > it a spin with tcmalloc I would be very much interested in the result. > > Thanks, > > Mathieu > -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com