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 15:37:01 +0100
From:	Alex Bennée <alex.bennee@...aro.org>
To:	Christoffer Dall <christoffer.dall@...aro.org>
Cc:	Peter Maydell <peter.maydell@...aro.org>,
	kvm-devel <kvm@...r.kernel.org>,
	arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
	"kvmarm\@lists.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


Christoffer Dall <christoffer.dall@...aro.org> writes:

> 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.

Certainly there are some cases where the kernel doesn't have all the
information. For example it doesn't know if the soft break was inserted
by the guest or the host. That to me favours the "let userspace deal
with the ugly" approach.

-- 
Alex Bennée
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