On Mon, Jun 03, 2019 at 03:03:48PM +0100, Andre Przywara wrote:
> On Mon, 3 Jun 2019 12:23:03 +0100
> Dave Martin <dave.mar...@arm.com> wrote:
>
> Hi Dave,
>
> > On Fri, May 31, 2019 at 06:04:16PM +0100, Andre Przywara wrote:
> > > On Thu, 30 May 2019 16:13:10 +0100
> > > Dave Martin <dave.mar...@arm.com> wrote:
> > >
> > > > From: Amit Daniel Kachhap <amit.kach...@arm.com>
> > > >
> > > > This patch adds a runtime capabality for KVM tool to enable Arm64 8.3
> > > > Pointer Authentication in guest kernel. Two vcpu features
> > > > KVM_ARM_VCPU_PTRAUTH_[ADDRESS/GENERIC] are supplied together to enable
> > > > Pointer Authentication in KVM guest after checking the capability.
> > > >
> > > > Command line options --enable-ptrauth and --disable-ptrauth are added
> > > > to use this feature. However, if those options are not provided then
> > > > also this feature is enabled if host supports this capability.
> > >
> > > I don't really get the purpose of two options, I think that's quite
> > > confusing. Should the first one either be dropped at all or called
> > > something with "force"?
> > >
> > > I guess the idea is to fail if pointer auth isn't available, but the
> > > option is supplied?
> > >
> > > Or maybe have one option with parameters?
> > > --ptrauth[,=enable,=disable]
> >
> > So, I was following two principles here, either or both of which may be
> > bogus:
> >
> > 1) There should be a way to determine whether KVM turns a given feature
> > on or off (instead of magically defaulting to something).
> >
> > 2) To a first approaximation, kvmtool should allow each major KVM ABI
> > feature to be exercised.
> >
> > 3) By default, kvmtool should offer the maximum feature set possible to
> > the guest.
> >
> >
> > (3) is well established, but (1) and (2) may be open to question?
> >
> > If we hold to both principles, it makes sense to have options
> > functionally equivalent to what I suggested (where KVM provides the
> > control in the first place), but there may be more convenient ways
> > to respell the options.
> >
> > If we really can't decide, maybe it's better to drop the options
> > altogether until we have a real use case.
>
> In general I prefer the lack of a *need* for options over tuneability, but my
> concern is not so much exposing this knob, but more how it's done ...
>
> > I've found the options very useful for testing and debugging on the SVE
> > side, but I can't comment on ptrauth. Maybe someone else has a view?
>
> Given that kvmtool was designed as a hacker tool, I find it quite useful to
> play around with those setting. I just have my gripes with those
> enable/disable pair, which are two related, but actually separate options,
> both polluting the command line options space and also being confusing to the
> user.
> I would be much happier if we would have one option per feature and a
> parameter: "--ptrauth={enable,disable}". Omitting the option altogether
> defaults to "enabled-if-available". Specifying it will force it on or off,
> accompanied by an error message if either(?) if not possible. This would also
> remove the need for the somewhat awkward "don't enable both" check.
> It would also more easily allow a common parser, to be used by both ptrauth
> and SVE, for instance.
> We could even introduce an explicit "default" parameter value, just in case
> people want to spell this case out.
>
> What do you think about this?
Happy to do something like that, though it looks like the decision to
drop the options altogether may preempt that...
Cheers
---Dave
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