Message-ID: <97b12c99-60e4-e3d9-a728-e7d1a7c09e41@intel.com>
Date:   Thu, 18 Nov 2021 14:19:31 -0800
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101
 Firefox/91.0 Thunderbird/91.3.1
Subject: Re: [RFC PATCH 00/13] x86 User Interrupts support
Content-Language: en-US
To:     Pavel Machek <pavel@ucw.cz>
CC:     <x86@kernel.org>, Tony Luck <tony.luck@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>,
        Jens Axboe <axboe@kernel.dk>,
        Christian Brauner <christian@brauner.io>,
        Peter Zijlstra <peterz@infradead.org>,
        Shuah Khan <shuah@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Jonathan Corbet <corbet@lwn.net>,
        Ashok Raj <ashok.raj@intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        "Gayatri Kammela" <gayatri.kammela@intel.com>,
        Zeng Guang <guang.zeng@intel.com>,
        "Dan Williams" <dan.j.williams@intel.com>,
        Randy E Witt <randy.e.witt@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        Ramesh Thomas <ramesh.thomas@intel.com>,
        <linux-api@vger.kernel.org>, <linux-arch@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <linux-kselftest@vger.kernel.org>
References: <20210913200132.3396598-1-sohil.mehta@intel.com>
 <20211001081903.GA18962@amd>
From:   Sohil Mehta <sohil.mehta@intel.com>
In-Reply-To: <20211001081903.GA18962@amd>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?cmh3R2JRZTF6RWY2ajM3aFFhSy90eURsUDR5K2xGNlhSUzRpOGFKclU3RnBM?=
 =?utf-8?B?MEVtVnNCWkhTbjVqeHc2T0R0dnZGY0Nuc0Rybm5pUHF0c0NXdkFOZ1habU03?=
 =?utf-8?B?WUN1dzQ2bWJKVEJEWmtSbUg3S2ZPTHBKYkZ4RVhIZGZ2RGlWaDhrTFFGbGpW?=
 =?utf-8?B?LzgvSHh1UUhUTVNlLzBodXhRZU4rK0N1SlZKUUdrOVUrZUg2ZDkwL0RPdnp1?=
 =?utf-8?B?eFRZR01YQ0dRUkYyUmRkTnpubzFvT2lEd0hobDNQNTdZVFVON1FXUkNSNXRE?=
 =?utf-8?B?OTMwaHFWVHJNWHM5SjhKUG1reHRMQW9icDE4VVZMVXlUSGFNc25JM0dyQnVh?=
 =?utf-8?B?REYzSzlOTGRKSXFCallMUkxUUTBQdER2VVNxbmdmamx5cnp1dUpkSml3ZjM0?=
 =?utf-8?B?UmwyZFdpcWpXcEYzQ0NBNndyVFY2MHlGdk1iQTU3dGhNRHFIY2ptWEpTZXpl?=
 =?utf-8?B?a0x1M3A1VUFnU0Q5UGdZRGdxZ1FKd0dKZDcwQkVCWS95ditqcVh3VWdkejRF?=
 =?utf-8?B?QUNEUkF5MmNIM2FnQmJVOUU5WHI5eFhuVG05VnBDNWQ4U0o0YkprRTBuR2Jv?=
 =?utf-8?B?TGdzNnVIWFQ0ZVc0UFhSM2lPQlJZaVlHSVQwV0UzbXlhRmxXOW9sMHRpSTVX?=
 =?utf-8?B?d2RaMFBTZWl0cHVIR2tkSmVvUlg2OWE3QVRYeEpMbjkyc0FMb1VBbG1DQk81?=
 =?utf-8?B?TE51NStobGk3ZUNOZE4rbWVYcTExZTFtMDdhR2k2Q1Q5YXVrWmw4QjFreWlq?=
 =?utf-8?B?Yy9Xcjgzc01BNEVwWnU2cTBFbzdwaU16MFNXaGw3RkpmRXM4Y2IvdG11RmJk?=
 =?utf-8?B?Vlp1QjBYbFpuUkRtN04xYkg1YkphN2VNZkxzOEdKWHBaaVhKbFNYaGlhZWpG?=
 =?utf-8?B?ekV0Z1NOdUJVdkNpZ29mT0pNUE5wd2ppckY1U0lCbGRjT083c1hScXVvdFBa?=
 =?utf-8?B?Y1JXa0FMbVRZSUNhZ21vbmRzNVEvLzdObFV1UlhmaDNadjdGWm5OZ290WkdO?=
 =?utf-8?B?U0RjV2Fzeit2N2pJQktIaUtNNkx5SEYwcy9ldGhpdnRJSXMyZ3JBNkEvbWFM?=
 =?utf-8?B?VzROM0w2dlhvcjdwTis1OE9FbG9MRzNZSXNoZDB1UU5KdzFOSCtMSXozazFO?=
 =?utf-8?B?KzlrWWFlS2o1cTRGbWJXSC9hZnpkWmoxb1VYcVg3MWw2bUFkT0tUNFp5bXV3?=
 =?utf-8?B?TzdodC83YTFjck1sUEQwc0FBWkFHZ2cycUk2VHgrMW5DWXRwNHNiNWNWeEVO?=
 =?utf-8?B?REwzUDVOQzFYOEoraVhEZHd4UnZCbUx2MzhOdjdNcnZMaG90WnE3OVVvdmFk?=
 =?utf-8?B?c3lrYXBqQlp5TWZYSFlwdXo3QUZTbFNPRE9qOCs2blFacGRrbTQ1bVkxY2FT?=
 =?utf-8?B?WUJqY25ZMWlSekJ3aThGNTE3c0NobEszbGp1N0pKNjlvVlVYRjVaaU82NVd0?=
 =?utf-8?B?NloweWhCQ0F6SkYwOTg4cS96V01Dc01OVWpQaU02cHVNd2dGM2xZMjdkZmds?=
 =?utf-8?B?ZTNHK1VqWHlqNWpFWTgwUHJmaTRYN0VJSmNGKzRxN243L0RFdTZFMnhnVExh?=
 =?utf-8?B?UWlCR3ZMbklqUStyL2NvWVBoL0ttdUpFRHA3MEo4OG9Jb2N6MExRUVpVNFhh?=
 =?utf-8?B?SFVKcXZta21SejBCbjQwMjhWVTRZRW1RYkoxZS9INjA2Qmo1QTNSZVFxZ3BO?=
 =?utf-8?B?VDYxQ0R5UzIwcEhaVGVhY1VzTVhCSWVkYUl4U1dtZ2wrQjRZZVc4ajVKTSsv?=
 =?utf-8?B?MXViL1lNOWd0Q2JMKzRMRlZRNzIvS1Zaa2ZBSm4vVlVtQTgyOGw1VDFobG13?=
 =?utf-8?B?OS82VDFPVTA0NzY2cE43bDc3S0h3MUczR3d6VGlJZnRic29SRFhUOFJINDdK?=
 =?utf-8?B?WnRibTBVTzVBd01sMEdtM3VaVFVkSDZHL1l2clhoZnRGOFhxTUsxSlNHRVhj?=
 =?utf-8?B?TEpIdkhDVEgxaUhWTTN4ckZPV0lQQ3BVNWFzdW55Z3JyY3NMT1RUUGp5ZjI1?=
 =?utf-8?B?a0Qvb3ZBcmg5OS83dUU1SHBoTnhhZTgrK3oxVjVqSE0xRXZ4RjNSckVEVW1o?=
 =?utf-8?B?Zzk2Tzh4cHcrS0dnMWVUQWIrRTBCdE9zT29hcTAyby84Z2YwbXp1aEl5bnRJ?=
 =?utf-8?B?L3VzTVczdW15WWRKbEJoYUNjdmdnU1hiSUREQm1FeUg4bmxhOWdnWUt3RmFY?=
 =?utf-8?Q?XkpxW9Sy8yJvddA1UBRpCOU=3D?=
X-MS-Exchange-CrossTenant-Network-Message-Id: a8de9bac-1437-4aea-75f1-08d9aae17ffa
X-MS-Exchange-CrossTenant-AuthSource: BYAPR11MB3320.namprd11.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 18 Nov 2021 22:19:34.3818
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 46c98d88-e344-4ed4-8496-4ed7712e255d
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: V4rfJQ0qjnkbdku8cOnAfFZ6DmQxK6Pbh9ttf8Mjd3RMrbgM37Oh1jTjKgB0q7icY3f3ykSij7M+06A9isGFBw==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR11MB3382
X-OriginatorOrg: intel.com
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 10/1/2021 1:19 AM, Pavel Machek wrote:
> Hi!
> 

Thank you for reviewing the patches!

>> Instructions
>> ------------
>> senduipi <index> - send a user IPI to a target task based on the UITT index.
>>
>> clui - Mask user interrupts by clearing UIF (User Interrupt Flag).
>>
>> stui - Unmask user interrupts by setting UIF.
>>
>> testui - Test current value of UIF.
>>
>> uiret - return from a user interrupt handler.
> 
> Are other CPU vendors allowed to implement compatible instructions?
> 
> If not, we should probably have VDSO entries so kernel can abstract
> differences between CPUs.
> 

Yes, we are evaluating VDSO support for this.

>> Untrusted processes
>> -------------------
>> The current implementation expects only trusted and cooperating processes to
>> communicate using user interrupts. Coordination is expected between processes
>> for a connection teardown. In situations where coordination doesn't happen
>> (say, due to abrupt process exit), the kernel would end up keeping shared
>> resources (like UPID) allocated to avoid faults.
> 
> Keeping resources allocated after process exit is a no-no.
> 

I meant the resource is still tracked via the shared file descriptor, so 
it will eventually get freed when the FD release happens. I am planning 
to include better documentation on lifetime rules of these shared 
resources next time.

Thanks,
Sohil