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

Christoffer Dall
2015-04-28T12:56:45Z

On Tue, Apr 28, 2015 at 10:34:12AM +0100, Peter Maydell wrote:
> On 28 April 2015 at 09:42, Alex Bennée <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