linux-kernel - Re: [PATCH 1/1] KVM: inject data abort if instruction cannot be decoded

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Message-ID: <CAFEAcA-mc6cLmRGdGNOBR0PC1f_VBjvTdAL6xYtKjApx3NoPgQ@mail.gmail.com>
Date:   Thu, 5 Sep 2019 09:16:54 +0100
From:   Peter Maydell <peter.maydell@...aro.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Heinrich Schuchardt <xypron.glpk@....de>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry@....com>,
        Suzuki K Pouloze <suzuki.poulose@....com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Daniel P . Berrangé <berrange@...hat.com>,
        arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] KVM: inject data abort if instruction cannot be decoded

On Thu, 5 Sep 2019 at 09:04, Marc Zyngier <maz@...nel.org> wrote:
> How can you tell that the access would fault? You have no idea at that
> stage (the kernel doesn't know about the MMIO ranges that userspace
> handles). All you know is that you're faced with a memory access that
> you cannot emulate in the kernel. Injecting a data abort at that stage
> is not something that the architecture allows.

To be fair, locking up the whole CPU (which is effectively
what the kvm_err/ENOSYS is going to do to the VM) isn't
something the architecture allows either :-)

> Of course, the best thing would be to actually fix the guest so that
> it doesn't use non-emulatable MMIO accesses. In general, that the sign
> of a bug in low-level accessors.

This is true, but the problem is that barfing out to userspace
makes it harder to debug the guest because it means that
the VM is immediately destroyed, whereas AIUI if we
inject some kind of exception then (assuming you're set up
to do kernel-debug via gdbstub) you can actually examine
the offending guest code with a debugger because at least
your VM is still around to inspect...

thanks
-- PMM