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



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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