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

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Message-ID: <3abdadab-9b7e-7cc0-5ec5-17fa385ce7d4@linaro.org>
Date:   Fri, 1 Oct 2021 12:41:59 -0400
From:   Richard Henderson <richard.henderson@...aro.org>
To:     Stefan Hajnoczi <stefanha@...hat.com>,
        Sohil Mehta <sohil.mehta@...el.com>,
        Peter Maydell <peter.maydell@...aro.org>,
        Alex Bennée <alex.bennee@...aro.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <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>, Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <christian@...uner.io>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Shuah Khan <shuah@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Raj Ashok <ashok.raj@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Gayatri Kammela <gayatri.kammela@...el.com>,
        Zeng Guang <guang.zeng@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Randy E Witt <randy.e.witt@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        Ramesh Thomas <ramesh.thomas@...el.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arch@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-kselftest@...r.kernel.org, qemu-devel@...gnu.org
Subject: Re: [RFC PATCH 00/13] x86 User Interrupts support

On 10/1/21 12:35 PM, Stefan Hajnoczi wrote:
> QEMU's TCG threads execute translated code. There are events that
> require interrupting these threads. Today a check is performed at the
> start of every translated block. Most of the time the check is false and
> it's a waste of CPU.
> 
> User interrupts can eliminate the need for checks by interrupting TCG
> threads when events occur.

We used to use interrupts, and stopped because we need to wait until the guest is in a 
stable state.  The guest is always in a stable state at the beginning of each TB.

See 378df4b2375.


r~