linux-kernel - Re: [PATCH 2/2] KVM: arm64: selftests: Disable single-step without relying on ucall()

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Message-ID: <Y3WF8YlvFm8NGYHd@google.com>
Date:   Thu, 17 Nov 2022 00:53:05 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        Reiji Watanabe <reijiw@...gle.com>
Subject: Re: [PATCH 2/2] KVM: arm64: selftests: Disable single-step without
 relying on ucall()

On Thu, Nov 17, 2022 at 12:23:50AM +0000, Sean Christopherson wrote:
> Automatically disable single-step when the guest reaches the end of the
> verified section instead of using an explicit ucall() to ask userspace to
> disable single-step.  An upcoming change to implement a pool-based scheme
> for ucall() will add an atomic operation (bit test and set) in the guest
> ucall code, and if the compiler generate "old school" atomics, e.g.

Off topic, but I didn't ask when we were discussing this issue. What is
the atomic used for in the pool-based ucall implementation?

>   40e57c:       c85f7c20        ldxr    x0, [x1]
>   40e580:       aa100011        orr     x17, x0, x16
>   40e584:       c80ffc31        stlxr   w15, x17, [x1]
>   40e588:       35ffffaf        cbnz    w15, 40e57c <__aarch64_ldset8_sync+0x1c>
> 
> the guest will hang as the local exclusive monitor is reset by eret,
> i.e. the stlxr will always fail due to the VM-Exit for the debug
> exception.

... due to the debug exception taken to EL2.

What's a VM-Exit anyways? ;-)

> Link: https://lore.kernel.org/all/20221006003409.649993-8-seanjc@google.com
> Cc: Oliver Upton <oliver.upton@...ux.dev>
> Cc: Marc Zyngier <maz@...nel.org>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

With the commit message nit:

Reviewed-by: Oliver Upton <oliver.upton@...ux.dev>

--
Thanks,
Oliver