linux-kernel - Re: [PATCH v5 08/24] sched: Introduce per memory space current virtual cpu id

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Date:   Mon, 14 Nov 2022 20:49:09 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api@...r.kernel.org, Christian Brauner <brauner@...nel.org>,
        Florian Weimer <fw@...eb.enyo.de>, David.Laight@...lab.com,
        carlos@...hat.com, Peter Oskolkov <posk@...k.io>,
        Alexander Mikhalitsyn <alexander@...alicyn.com>,
        Chris Kennelly <ckennelly@...gle.com>
Subject: Re: [PATCH v5 08/24] sched: Introduce per memory space current
 virtual cpu id

On Fri, Nov 11, 2022, Mathieu Desnoyers wrote:
> On 2022-11-10 23:41, Andy Lutomirski wrote:
> > On Thu, Nov 3, 2022 at 1:05 PM Mathieu Desnoyers
> > <mathieu.desnoyers@...icios.com> wrote:
> > Also, in my mind "virtual cpu" is vCPU, which this isn't.  Maybe
> > "compacted cpu" or something?  It's a strange sort of concept.
> 
> I've kept the same wording that has been introduced in 2011 by Paul Turner
> and used internally at Google since then, although it may be confusing if
> people expect kvm-vCPU and rseq-vcpu to mean the same thing. Both really end
> up providing the semantic of a virtually assigned cpu id (in opposition to
> the logical cpu id on the system), but this is much more involved in the
> case of KVM.

I had the same reaction as Andy.  The rseq concepts don't worry me so much as the
existence of "vcpu" in mm_struct/task_struct, e.g. switch_mm_vcpu() when switching
between KVM vCPU tasks is going to be super confusing.  Ditto for mm_vcpu_get()
and mm_vcpu_put() in the few cases where KVM currently does mmget()/mmput().