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SubjectRe: [PATCH 6/6] s390: introduce execute-trampolines for branches
On Wed, 7 Feb 2018 11:07:26 +0100
Pavel Machek <pavel@ucw.cz> wrote:

> On Wed 2018-02-07 08:00:11, Martin Schwidefsky wrote:
> > Add CONFIG_EXPOLINE to enable the use of the new -mindirect-branch= and
> > -mfunction_return= compiler options to create a kernel fortified against
> > the specte v2 attack.
> >
> > With CONFIG_EXPOLINE=y all indirect branches will be issued with an
> > execute type instruction. For z10 or newer the EXRL instruction will
> > be used, for older machines the EX instruction. The typical indirect
> > call
> >
> > basr %r14,%r1
> >
> > is replaced with a PC relative call to a new thunk
> >
> > brasl %r14,__s390x_indirect_jump_r1
> >
> > The thunk contains the EXRL/EX instruction to the indirect branch
> >
> > __s390x_indirect_jump_r1:
> > exrl 0,0f
> > j .
> > 0: br %r1
> >
> > The detour via the execute type instruction has a performance impact.
> > To get rid of the detour the new kernel parameter "nospectre_v2" and
> > "spectre_v2=[on,off,auto]" can be used. If the parameter is specified
> > the kernel and module code will be patched at runtime.
>
> This is really unfortunate naming of kernel option.
>
> spectre_v2=off sounds like we are turning the "bug" off, but i somehow
> suspect you are turning the bug _workaround_ off.

Well, that is the 1:1 copy of the x86 option. Do you want to change that
one as well?

--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.

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