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From:   Florian Weimer <fweimer@redhat.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
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Subject: Re: [PATCH v2 23/39] x86: Introduce userspace API for CET enabling
References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
        <20220929222936.14584-24-rick.p.edgecombe@intel.com>
        <87v8os0wx5.fsf@oldenburg.str.redhat.com>
        <8599719452d9615235f7fdd274a9b6ea04ab1f7c.camel@intel.com>
Date:   Wed, 12 Oct 2022 14:18:52 +0200
In-Reply-To: <8599719452d9615235f7fdd274a9b6ea04ab1f7c.camel@intel.com> (Rick
        P. Edgecombe's message of "Mon, 10 Oct 2022 16:28:57 +0000")
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* Rick P. Edgecombe:

> On Mon, 2022-10-10 at 12:56 +0200, Florian Weimer wrote:
>> > +     /* Only support enabling/disabling one feature at a time. */
>> > +     if (hweight_long(features) > 1)
>> > +             return -EINVAL;
>> 
>> This means we'll soon need three extra system calls for x86-64
>> process
>> start: SHSTK, IBT, and switching off vsyscall emulation.  (The latter
>> does not need any special CPU support.)
>> 
>> Maybe we can do something else instead to make the strace output a
>> little bit cleaner?
>
> In previous versions it supported enabling multiple features in a
> single syscall. Thomas Gleixner pointed out that (this was on the LAM
> patchset that shared the interface at the time) it makes the behavior
> of what to do when one feature fails to enable complicated:
>
> https://lore.kernel.org/lkml/87zgjjqico.ffs@tglx/

Can we return the bits for the features that were actually enabled?
Those three don't have cross-dependencies in the sense that you would
only use X & Y together, but not X or Y alone.

Thanks,
Florian