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Date:   Thu, 20 Jan 2022 12:03:40 +0000
From:   Mark Rutland <mark.rutland@arm.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Christian Borntraeger <borntraeger@linux.ibm.com>,
        linux-kernel@vger.kernel.org,
        Michael Ellerman <mpe@ellerman.id.au>,
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        Anup Patel <anup@brainfault.org>,
        Atish Patra <atishp@atishpatra.org>
Subject: Re: [PATCH v2 0/7] kvm: fix latent guest entry/exit bugs
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References: <20220119105854.3160683-1-mark.rutland@arm.com>
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On Thu, Jan 20, 2022 at 12:28:09PM +0100, Paolo Bonzini wrote:
> On 1/19/22 20:22, Mark Rutland wrote:
> > I wonder, is the s390 guest entry/exit*preemptible*  ?
> > 
> > If a timer IRQ can preempt in the middle of the EQS, we wouldn't balance
> > things before a ctx-switch to the idle thread, which would then be able
> > to hit this.
> > 
> > I'll need to go audit the other architectures for similar.
> 
> They don't enable interrupts in the entry/exit path so they should be okay.

True.

So it sounds like for s390 adding an explicit preempt_{disable,enable}() is the
right thing to do. I'll add that and explanatory commentary.

> RISC-V and x86 have an explicit preempt_disable/enable, while MIPS only has
> local_irq_disable/enable.
> 
> (MIPS is a mess of dead code, I have patches to clean it up).

Sure; I haven't wrapped my head around ppc yet, but I assume they keep
interrupts disabled as with the other simple cases.

Thanks,
Mark.