Date: Tue, 28 Apr 2015 14:56:45 +0200
From: Christoffer Dall <christoffer.dall@linaro.org>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>,
        kvm-devel <kvm@vger.kernel.org>,
        arm-mail-list <linux-arm-kernel@lists.infradead.org>,
        "kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>,
        Marc Zyngier <marc.zyngier@arm.com>, Alexander Graf <agraf@suse.de>,
        Andrew Jones <drjones@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>,
        Zhichao Huang <zhichao.huang@linaro.org>,
        "J. Kiszka" <jan.kiszka@siemens.com>,
        David Hildenbrand <dahi@linux.vnet.ibm.com>,
        Bharat Bhushan <r65777@freescale.com>, bp@suse.de,
        Gleb Natapov <gleb@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
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        Will Deacon <will.deacon@arm.com>,
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Subject: Re: [PATCH v2 06/10] KVM: arm64: guest debug, add SW break point
 support
Message-ID: <20150428125645.GA4137@cbox>
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On Tue, Apr 28, 2015 at 10:34:12AM +0100, Peter Maydell wrote:
> On 28 April 2015 at 09:42, Alex Benn?e <alex.bennee@linaro.org> wrote:
> > Peter Maydell <peter.maydell@linaro.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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