Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <77543974-cd67-3999-103e-6714d04f0e5e@efficios.com>
Date:   Fri, 23 Sep 2022 09:46:15 -0400
User-Agent: Mozilla/5.0 (X11; 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 <ckennelly@google.com>
References: <20220922105941.237830-1-mathieu.desnoyers@efficios.com>
 <e753568d-599c-d81a-8456-085bbbb0264d@efficios.com>
 <CAEE+ybnLUHjU5-dWcWgcWiq-AM4ocquSbZ=PWiuexEsPB8P5Gw@mail.gmail.com>
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Paul Turner <pjt@google.com>, Peter Oskolkov <posk@posk.io>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "carlos@redhat.com" <carlos@redhat.com>,
        Florian Weimer <fw@deneb.enyo.de>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>
In-Reply-To: <CAEE+ybnLUHjU5-dWcWgcWiq-AM4ocquSbZ=PWiuexEsPB8P5Gw@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?T21BV201Z1I2eHc5RE5aQXphaEluZkl6WDBkejVHTGhiMThJYzRUeUFoNjNY?=
 =?utf-8?B?VzVyU2xjL1NINzBVa05EenoweWNMWHVsSzRjQlgrb2lRNWovR0JqZCtnWjY1?=
 =?utf-8?B?WUNtM212Rnk0UER6Q2kwZytScG81aHFkaTZHNXpVcGMrcHZYNzNsQnA0ak51?=
 =?utf-8?B?UnVvbzZxRVVTVy9PamZELzNPV0crUmN0K2VUaFdTdm5lK0ZJZC9mMkk3Mkd0?=
 =?utf-8?B?TENSUVFCdXFyR3JmV0tWWnQ1ZmgxRjNtR1hoS25ObnV6bkYzRWVFTTlqZlZq?=
 =?utf-8?B?cGt2bUdSZzM3VHoxNTB4dWFkT0d5b3hLRE5nQU9BOWd3YnR4QWJMRk5VR2VV?=
 =?utf-8?B?dVdyRTJ3N2w4WkxXa2Y0VkhUeW8vZk9MWmJ3YUJFeWRwZXpUQzJjdmZBaFFl?=
 =?utf-8?B?OFgvZFlOUHk3YUNaR0tLcXRnQkJhRXgrVnpSSDY0ZmpxNmRDN2IrOVBBWEhp?=
 =?utf-8?B?OG5GMDVCWW91MXRSZnFaaGNvYWRJbDhvV0QrTlYrbnIvWi9hd3owQWVPRVM2?=
 =?utf-8?B?WFUvZXZhVzNkKzByNU1ROFBvQ3MwTGV6VWFBaUV3M2JxaVRSSTVXWWxuMEF1?=
 =?utf-8?B?dEp5UXZUTkRjY0JnY1l1S2QranNmVmhOQ0pmWk05ZUZMQ1hlRTVERVJidGl3?=
 =?utf-8?B?VHczNWQzMHIxVWEyRFY0OFRYMTl3ZHRwTHVRRUFWTzBGVG9UL3gxKysrU1pI?=
 =?utf-8?B?ZmxQdU9DK0RvYVNhTS9BRU1TbW9RVGxGOWtpR3dJejZzYWZKOFdXT3gzdDZV?=
 =?utf-8?B?S3pMS29sUmNMcjVFWFF6V3BoOE5EWkJreXFlQWJYNmlpWFJHdm5xYTR4b1Nn?=
 =?utf-8?B?ZWxTdThLeUpNTFAxdzdTdklsREkxRDFiOUd4TEg2T3BRbkFwM0tJNHdMdURV?=
 =?utf-8?B?ZC9GRDNaT1F2ZEJETmQ0UVlqQXNBTkdGVTZRZ0F1S0Q2MW94MENWeUJXL3Mr?=
 =?utf-8?B?QU1lMzdkOVQ1aGNTRFVXU0tHbmdLZkRyN2g2djRFV2h1d09lQ212OGV2M0Ev?=
 =?utf-8?B?RmMyNXhjK1dadVAzOFRBRk1UVk1MdXlSc01ZUThEVWxUTGsyWCtYNzVSWS9I?=
 =?utf-8?B?SjFzNXJHeGtLcTBDeGlDSHhnMnh0bzdaajdNeXQzS3hCc3NhdklnVm5DWm9Z?=
 =?utf-8?B?bXliZEJ1OTdNT1hFYWVuS0lEWHUrdnpiRkdJdEwwYTJDRjhxYXhuM0N6cnVt?=
 =?utf-8?B?c0JqMHZDbktMcUs3TTJzcDVFVEpJSi9IekxmaEUzazkrR1NxWWNIWkxrVXlX?=
 =?utf-8?B?bFdrYVl4cmRLTG1QVHVVUlcvZURWUjVOUlFLUklReHU3UFdLWUNuWVpwemx6?=
 =?utf-8?B?Z3RxY3J4UC9MMVdJaVgrL2lvT3dIZmFibW84SldWMHJwTU9ZTkZpeCszeWNx?=
 =?utf-8?B?QTJROWEvM2tOd2NMTittVFBaME1aYnNiY3dmdERueE43VG9Mc2V1a0FFUGtl?=
 =?utf-8?B?NHFlNlM2bzlmTWtPbkNBeFBGWWJwNUF6L1IwNzlGZWorM0FDdWpWVmhhQlZ5?=
 =?utf-8?B?Z24rNE9MSlA4MldtM1UzSXR3ZWVwM3B0dXcvVy9tZVNTaHk5OTRJWlYrMFpm?=
 =?utf-8?B?VUdla1hTaXBOUjQ0d05oQXgwUVQzVVA3dHEvTmhPZ0dSd2pQRlVPeWpmdERX?=
 =?utf-8?B?QmphclY5YjNrZUdYSDhNU1JjU096THNkQXpFQkpZdDBKazd0d3pROVJaZHhH?=
 =?utf-8?B?VmJiTGxmdDJFeU8wTnRLS2pvMGViRTVrR3hzb3ZaVEJJS0NSRHRFbHNmd1lz?=
 =?utf-8?B?azJ3dG16Y254UWk0QzUzbXFOalRjY1I0U3laYnR0MUNXTmUzZjFFdGtFdmRS?=
 =?utf-8?B?bkVUUWdXL2IyaGFBdVEvckhYUG0rbEtQZGJTc3YyMzIwVHVrTVZ0ZTFwZHhU?=
 =?utf-8?B?dDg3ZmpTZ094NjNQWTB4Ym45bSsvcSs2bFNjYmpBZWYwakVPVVFXaVBaZlN1?=
 =?utf-8?B?eXlEQVA3TXk3SldPVGJuUW5FejkweGRTMVNFTnVaSTBkcnBJdkViNXdiR1Rx?=
 =?utf-8?B?eTA4dWNzb0ZkNVpocXNoM1lxb3FrZkVQM1NNS3RKWTVZcDZWOXNNa2NTQjVj?=
 =?utf-8?B?UTVwWkVMTkpjdnI1YitzZktxQmpZYkNFbVNZbExYUyt1VEx5aDUrTzc5ZE9Z?=
 =?utf-8?B?QUpSdWczQnFRRk55azdCcFdoeXZBQVdUdXMzc0xuc3gwTnRPNGxvNk5yVlVO?=
 =?utf-8?Q?8nVD6XpM0asSJ7vTvjzUzUE=3D?=
X-OriginatorOrg: efficios.com
X-MS-Exchange-CrossTenant-Network-Message-Id: f7964e4f-d1b3-4123-6300-08da9d69e056
X-MS-Exchange-CrossTenant-AuthSource: YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 23 Sep 2022 13:45:28.8168
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 4f278736-4ab6-415c-957e-1f55336bd31e
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: MCGZPO4URcidHjTdaaREQJAhrmT+H0vha0KiTQGQAb3ajTTBMWYOL3eMViSQIP4fN43KDRazk37M9dWH93Uq1beggruS+I8ySu3uyUhN3OY=
X-MS-Exchange-Transport-CrossTenantHeadersStamped: YQBPR0101MB5687
X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIM_SIGNED,
        DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,NICE_REPLY_A,RCVD_IN_DNSWL_NONE,
        RCVD_IN_MSPIKE_H2,SPF_HELO_PASS,SPF_PASS autolearn=ham
        autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on
        lindbergh.monkeyblade.net
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 2022-09-22 16:10, Chris Kennelly wrote:
> Hi,
> 
> I still need to update the code in TCMalloc to cooperate with the new 
> glibc ABI/convention.  One concern I have is that it looks like I might 
> need to add a extra memory dereference (or two) to get the early 
> initialized offsets provided by glibc folded into the read of the cpu_id 
> field.

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 
> <mathieu.desnoyers@efficios.com <mailto:mathieu.desnoyers@efficios.com>> 
> 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