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

From: Mark Rutland <mark.rutland@arm.com>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: "Alex Bennée" <alex.bennee@linaro.org>,
	kvm-devel <kvm@vger.kernel.org>,
	"Marc Zyngier" <marc.zyngier@arm.com>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Will Deacon" <will.deacon@arm.com>,
	"open list" <linux-kernel@vger.kernel.org>,
	"Christoffer Dall" <christoffer.dall@linaro.org>,
	kvmarm@lists.cs.columbia.edu,
	arm-mail-list <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC PATCH] KVM: arm64: don't single-step for non-emulated faults
Date: Fri, 9 Nov 2018 13:29:56 +0000	[thread overview]
Message-ID: <20181109132955.kqfccmqrugfj5rkl@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <CAFEAcA-xuUPukf9EPeetr_+RSU7FPDMXmyk3zbjY=FcV65CF=g@mail.gmail.com>

On Fri, Nov 09, 2018 at 12:56:54PM +0000, Peter Maydell wrote:
> On 9 November 2018 at 12:49, Mark Rutland <mark.rutland@arm.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.