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

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Date:   Fri, 9 Nov 2018 13:29:56 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Maydell <peter.maydell@...aro.org>
Cc:     Alex Bennée <alex.bennee@...aro.org>,
        kvm-devel <kvm@...r.kernel.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        open list <linux-kernel@...r.kernel.org>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        kvmarm@...ts.cs.columbia.edu,
        arm-mail-list <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH] KVM: arm64: don't single-step for non-emulated faults

On Fri, Nov 09, 2018 at 12:56:54PM +0000, Peter Maydell wrote:
> On 9 November 2018 at 12:49, Mark Rutland <mark.rutland@....com> wrote:
> > I'm not saying anything about *decisions*. I'm saying that we can make
> > the state consistent by advancing the singlestep state in the same way
> > that HW does, at the instant it advances the PC.
> >
> > i.e. do that in kvm_skip_instr(), as I've done in my local tree.
> >
> > That mirrors the HW, and we don't need to special-case any handling for
> > emulated vs non-emulated instructions.
> 
> You also need to do it in the "set PC because we're making the guest
> take an exception" code path, which doesn't go through kvm_skip_instr().

Sure.

> This corresponds to the two kinds of "step completed" in hardware as
> noted in DDI0487D.a D2.12.3 fig D2-3 footnote b:
>  * executing the instruction to be stepped without taking an exception
>  * taking an exception to an exception level that debug exceptions
>    are enabled from [ie guest EL1 in our case]

Thanks for the pointer!

Mark.