linux-kernel - Re: Possible nohz-full/RCU issue in arm64 KVM

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Message-ID: <b8e273bf-ae2c-cc5c-0d20-33f3dd12053c@redhat.com>
Date:   Tue, 4 Jan 2022 18:07:32 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        paulmck <paulmck@...nel.org>, maz <maz@...nel.org>,
        frederic <frederic@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        rcu <rcu@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Anup Patel <Anup.Patel@....com>
Subject: Re: Possible nohz-full/RCU issue in arm64 KVM

On 1/4/22 17:39, Mark Rutland wrote:
> My main issue here was just that it's really difficult to see how the
> entry/exit logic is balanced, and I reckon we can solve that by splitting
> guest_{enter,exit}_irqoff() into helper functions to handle the vtime
> accounting separately from the context tracking, so that arch code can do
> something like:
> 
>    guest_timing_enter_irqoff();
>    
>    guest_eqs_enter_irqoff();
>    < actually run vCPU here >
>    guest_eqs_exit_irqoff();
>    
>    < handle pending IRQs here >
>    
>    guest_timing_exit_irqoff();
> 
> ... which I hope should work for RISC-V too.
> 
> I've had a go, and I've pushed out a WIP to:
> 
>    https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/kvm/rcu

Yes, you have a point and it makes sense for x86 too.  You can send me a 
topic branch once you get all the acks.  Thanks!

Paolo