linux-kernel - Re: [RFC PATCH 00/13] x86 User Interrupts support

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Date:   Thu, 18 Nov 2021 14:19:31 -0800
From:   Sohil Mehta <sohil.mehta@...el.com>
To:     Pavel Machek <pavel@....cz>
CC:     <x86@...nel.org>, Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <christian@...uner.io>,
        Peter Zijlstra <peterz@...radead.org>,
        Shuah Khan <shuah@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Ashok Raj <ashok.raj@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "Gayatri Kammela" <gayatri.kammela@...el.com>,
        Zeng Guang <guang.zeng@...el.com>,
        "Dan Williams" <dan.j.williams@...el.com>,
        Randy E Witt <randy.e.witt@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Ramesh Thomas <ramesh.thomas@...el.com>,
        <linux-api@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>
Subject: Re: [RFC PATCH 00/13] x86 User Interrupts support

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