Re: [PATCH v2 5/8] x86/speculation: Add basic support for IBPB

[David Woodhouse]

On Sun, 2018-01-21 at 20:54 +0100, Borislav Petkov wrote:
> On Sun, Jan 21, 2018 at 07:31:39PM +0000, David Woodhouse wrote:
> > > ÂÂÂÂÂÂÂÂif (boot_cpu_has(X86_FEATURE_IBPB))
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂwrmsr(MSR_IA32_PRED_CMD, PRED_CMD_IBPB, 0);
> > >Â
> > > Problem solved.
> >Â
> > Nope. Plenty of patch sets *did* have the bug above though, until it
> > was spotted.
> 
> And that bug is...?

That bug is the *reason* we're arguing about static_cpu_has vs.
ALTERNATIVE.Â

A conditional branch that the CPU sees can be speculated over...

Now, Andrew is right that in a number of cases there will be another
serialising instruction before we ever hit a problematic indirect
branch. But as I just said elsewhere, I'd really like the *primitives*
to support unconditional operation.

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