linux-kernel - Re: [PATCH v2 06/10] KVM: arm64: guest debug, add SW break point support

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Date:	Tue, 28 Apr 2015 14:56:45 +0200
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Peter Maydell <peter.maydell@...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>,
	Andrew Jones <drjones@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Zhichao Huang <zhichao.huang@...aro.org>,
	"J. Kiszka" <jan.kiszka@...mens.com>,
	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	Bharat Bhushan <r65777@...escale.com>, bp@...e.de,
	Gleb Natapov <gleb@...nel.org>,
	Jonathan Corbet <corbet@....net>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 06/10] KVM: arm64: guest debug, add SW break point
 support

On Tue, Apr 28, 2015 at 10:34:12AM +0100, Peter Maydell wrote:
> On 28 April 2015 at 09:42, Alex Bennée <alex.bennee@...aro.org> wrote:
> > Peter Maydell <peter.maydell@...aro.org> writes:
> >> Does the kernel already have a conveniently implemented "inject
> >> exception into guest" lump of code? If so it might be less effort
> >> to do it that way round, maybe.
> >
> > So you pointed out we can't just re-inject the exceptions we get as we
> > need to map from things like ESR_ELx_EC_WATCHPT_LOW to
> > ESR_ELx_EC_WATCHPT_CUR before re-injection.
> >
> > Of course if it is as simple as modifying the ESR_EL1 register and
> > returning +ve in the handle_exit path then I can do that but I assumed
> > if any other wrangling needs doing it should be done in userspace.
> 
> Well, somebody's got to do it, and it's the same amount of work
> either way (fiddling with ESR, making sure we direct the guest
> to the right exception vector entry point, maybe a few other
> things).
> 
We already have code in the kernel to inject data/instruction aborts,
but not sure how much benefit there is in re-using that.  It's up to you
really, but I think the kernel code should be clear about what the
intention is so that we don't end up in a situation where: (1) The
intended behavior is unclear/vague, and (2) it doesn't actually work in
practice so nobody can follow the code.

Thanks,
-Christoffer
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