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

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Message-ID: <ada9b2aa-f1e7-d566-0384-002890441a30@redhat.com>
Date:   Fri, 17 Dec 2021 15:51:14 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Mark Rutland <mark.rutland@....com>,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     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>,
        "kvm-riscv@...ts.infradead.org" <kvm-riscv@...ts.infradead.org>,
        Anup Patel <Anup.Patel@....com>
Subject: Re: Possible nohz-full/RCU issue in arm64 KVM

On 12/17/21 14:21, Mark Rutland wrote:
> I'm not immediately sure what we*should*  do here. What does x86 do for an IRQ
> taken from a guest mode? I couldn't spot any handling of that case, but I'm not
> familiar enough with the x86 exception model to know if I'm looking in the
> right place.

ARM is missing something like commit 160457140187 ("KVM: x86: Defer 
vtime accounting 'til after IRQ handling", 2021-05-05).

With that change, it would be possible to move guest_exit() in the 
irq-disabled region without breaking time accounting.

RISC-V has the same issue and it would be fixed in the same way, so 
let's Cc Anup too.

Paolo