Re: [Xen-devel] [PATCH for-4.13 2/2] xen/nospec: Introduce CONFIG_SPECULATIVE_BRANCH_HARDEN and disable it

[Julien Grall <julien.grall@xxxxxxx>]

Hi Andrew,

On 01/10/2019 10:41, Andrew Cooper wrote:
> On 01/10/2019 10:26, Julien Grall wrote:
> > Hi Jan,
> >
> > On 01/10/2019 10:22, Jan Beulich wrote:
> > > On 01.10.2019 11:17, Julien Grall wrote:
> > > > On 01/10/2019 00:21, Andrew Cooper wrote:
> > > > > On 30/09/2019 21:17, Julien Grall wrote:
> > > > > > My worry is this gate config gate nothing on Arm so the user may have
> > > > > > a false sense that it can be used (even though it is clearly BROKEN).
> > > > > >
> > > > > > Also the name is quite close to the CONFIG_HARDEN_PREDICTOR on Arm
> > > > > > and
> > > > > > may confuse user. Although, I don't have a better name so far :/
> > > > > The "depends on BROKEN" means it will never show up to a user in
> > > > > menuconfig, which is why it was only CC to x86, and not to rest.
> > > > What's the long term plan for this option? Are you planning to
> > > > remove it
> > > > completely or just the dependencies on BROKEN?
> > > >
> > > > My concern is if this option will ever become selectable then it
> > > > will not doing
> > > > what's expected on Arm.
> > > >
> > > > So, even if in principle it is arch agnostic, it feels to me this
> > > > option should
> > > > better be implemented in x86/Kconfig.
> > > I don't think so, no. When BROKEN is to be removed, it ought to be
> > > replaced by a suitable "depends on HAVE_*", which Arm could choose
> > > to not select.
> > So there are an expectation this option will be used by common code in
> > the future?
> It already is.  ARM has stubs for evaluate_nospec() etc.
>
> My gut feeling is that the only way this is going to be resolved sanely
> is with a compiler feature or plugin, at which point it reasonably can
> be cross-arch.
Fair enough. I don't have any more concern then. I will have a think about the 
naming or the possibly to merge the two configs (the common and arm ones) together.
Cheers,

--
Julien Grall

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