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Date:   Tue, 28 Sep 2021 20:30:27 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Sohil Mehta" <sohil.mehta@intel.com>,
        "the arch/x86 maintainers" <x86@kernel.org>
Cc:     "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>, "Jens Axboe" <axboe@kernel.dk>,
        "Christian Brauner" <christian@brauner.io>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        "Shuah Khan" <shuah@kernel.org>, "Arnd Bergmann" <arnd@arndb.de>,
        "Jonathan Corbet" <corbet@lwn.net>,
        "Raj Ashok" <ashok.raj@intel.com>,
        "Jacob Pan" <jacob.jun.pan@linux.intel.com>,
        "Gayatri Kammela" <gayatri.kammela@intel.com>,
        "Zeng Guang" <guang.zeng@intel.com>,
        "Williams, Dan J" <dan.j.williams@intel.com>,
        "Randy E Witt" <randy.e.witt@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        "Ramesh Thomas" <ramesh.thomas@intel.com>,
        "Linux API" <linux-api@vger.kernel.org>,
        linux-arch@vger.kernel.org,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
        linux-kselftest@vger.kernel.org
Subject: Re: [RFC PATCH 11/13] x86/uintr: Introduce uintr_wait() syscall
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On Mon, Sep 13, 2021, at 1:01 PM, Sohil Mehta wrote:
> Add a new system call to allow applications to block in the kernel and
> wait for user interrupts.
>

...

>
> When the application makes this syscall the notification vector is
> switched to a new kernel vector. Any new SENDUIPI will invoke the kernel
> interrupt which is then used to wake up the process.

Any new SENDUIPI that happens to hit the target CPU's ucode at a time when the kernel vector is enabled will deliver the interrupt.  Any new SENDUIPI that happens to hit the target CPU's ucode at a time when a different UIPI-using task is running will *not* deliver the interrupt, unless I'm missing some magic.  Which means that wakeups will be missed, which I think makes this whole idea a nonstarter.

Am I missing something?