linux-kernel - Re: [RFC PATCH] KVM: arm64: don't single-step for non-emulated faults

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Message-ID: <87sh0b69hz.fsf@linaro.org>
Date:   Thu, 08 Nov 2018 12:26:16 +0000
From:   Alex Bennée <alex.bennee@...aro.org>
To:     Peter Maydell <peter.maydell@...aro.org>
Cc:     kvm-devel <kvm@...r.kernel.org>,
        arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] KVM: arm64: don't single-step for non-emulated faults


Peter Maydell <peter.maydell@...aro.org> writes:

> On 7 November 2018 at 17:39, Peter Maydell <peter.maydell@...aro.org> wrote:
>> On 7 November 2018 at 17:10, Alex Bennée <alex.bennee@...aro.org> wrote:
>>> Not all faults handled by handle_exit are instruction emulations. For
>>> example a ESR_ELx_EC_IABT will result in the page tables being updated
>>> but the instruction that triggered the fault hasn't actually executed
>>> yet. We use the simple heuristic of checking for a changed PC before
>>> seeing if kvm_arm_handle_step_debug wants to claim we stepped an
>>> instruction.
>>>
>>> Signed-off-by: Alex Bennée <alex.bennee@...aro.org>
>>
>> What's the rationale for this change? Presumably it's fixing
>> something, but the commit message doesn't really say what...
>>
>> This feels to me like it's working around the fact that
>> we've separated two things ("advance pc (or set it if we're
>> going to make the guest take an exception)" and "notice that
>> we have completed a single step") that should be handled
>> at one point in the code.
>
> ...so for instance if your guest PC is at the entrypoint for
> an exception, and you singlestep and take the same exception
> again, this should count as a single step completed, even
> though the PC has not changed. Granted, that's a little
> contrived, but it can happen in cases where the guest gets
> completely confused and is sitting in a tight loop taking
> exceptions because there's no ram at the vector table
> address, or whatever.

The alternative I thought of as I was hacking^H^H^H^H^H^H carefully
engineering this was to expand arm_exit_handlers[] and tag each handler
that was an instruction emulation and gate on that.

--
Alex Bennée