Re: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature

[Andrea Arcangeli <aarcange@redhat.com>]

On Thu, Jan 04, 2018 at 07:33:45PM +0100, Borislav Petkov wrote:
> On Thu, Jan 04, 2018 at 09:56:47AM -0800, Tim Chen wrote:
> > There are 2 ways to control IBRS
> > 
> > 1. At boot time
> >     noibrs kernel boot parameter will disable IBRS usage
> > 
> > Otherwise if the above parameters are not specified, the system
> > will enable ibrs and ibpb usage if the cpu supports it.
> > 
> > 2. At run time
> >     echo 0 > /sys/kernel/debug/ibrs_enabled will turn off IBRS
> >     echo 1 > /sys/kernel/debug/ibrs_enabled will turn on IBRS in kernel
> >     echo 2 > /sys/kernel/debug/ibrs_enabled will turn on IBRS in both userspace and kernel
> 
> I am not sure that tristate is really needed. What's wrong with on/off
> only?

well, tristate is to provide a new feature (which is also incidentally
the mode new silicon will prefer to run).

> Also, stuff like that:
> 
> +/* mutex to serialize IBRS control changes */
> +DEFINE_MUTEX(spec_ctrl_mutex);
> +EXPORT_SYMBOL(spec_ctrl_mutex);
> 
> looks ugly.

Consolidating in arch/x86/kernel/spec_ctrl.c would allow removing that
export.

Here I've got:

static DEFINE_MUTEX(spec_ctrl_mutex);

> Wrapper functions which everything calls and they hide internals are
> much more preferrable.

Agreed.

> And then, if at all, this needs to be connected to the retpolines fun,
> methinks, so that it can be decided at boot what to use.

Turning this off at any time is very easy, making reptoline runtime
disabled is more difficult.

Thanks,
Andrea