linux-kernel - Re: [PATCH 5/7] KVM: arm64: guest debug, add support for single-step

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Date:	Sun, 30 Nov 2014 10:20:19 +0000
From:	Peter Maydell <peter.maydell@...aro.org>
To:	Christoffer Dall <christoffer.dall@...aro.org>
Cc:	Alex Bennée <alex.bennee@...aro.org>,
	kvm-devel <kvm@...r.kernel.org>,
	arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	Marc Zyngier <marc.zyngier@....com>,
	Alexander Graf <agraf@...e.de>,
	"J. Kiszka" <jan.kiszka@...mens.com>,
	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	Bharat Bhushan <r65777@...escale.com>, bp@...e.de,
	Paolo Bonzini <pbonzini@...hat.com>,
	Gleb Natapov <gleb@...nel.org>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:ABI/API" <linux-api@...r.kernel.org>
Subject: Re: [PATCH 5/7] KVM: arm64: guest debug, add support for single-step

On 30 November 2014 at 10:10, Christoffer Dall
<christoffer.dall@...aro.org> wrote:
> In any case, I think it was related to how userspace observes the state
> of the CPU, because when you do the MMIO operation emulation in
> userspace, currently if you observe the PC though GET_ONE_REG, you'll
> see a PC pointing to the next instruction, not the one you're emulating
> which is strange.

Also if we ever add support for userspace to say "this MMIO should
fault" then we definitely need the PC-advance to happen afterwards,
not before.

> Not sure what the relation to a guest single-stepping itself was.

I think it just came up in the course of that discussion, because
single-step handling also needs to perform an action (clear PSTATE.SS)
as part of the "advance over this insn" operation. But I think that
you're right that doing the advance before dropping out to userspace
is no worse for singlestep than it is for any other case.

-- PMM
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