Re: [PATCH v3 3/6] x86/bugs: Fix RSB clearing in indirect_branch_prediction_barrier()

[Josh Poimboeuf]

On Fri, Apr 04, 2025 at 05:45:37PM +0300, Nikolay Borisov wrote:
> 
> 
> On 2.04.25 г. 21:19 ч., Josh Poimboeuf wrote:
> > IBPB is expected to clear the RSB.  However, if X86_BUG_IBPB_NO_RET is
> > set, that doesn't happen.  Make indirect_branch_prediction_barrier()
> > take that into account by calling __write_ibpb() which already does the
> > right thing.
> 
> I find this changelog somewhat dubious. So zen < 4 basically have
> IBPB_NO_RET, your patch 2 in this series makes using SBPB for cores which
> have SRSO_NO or if the mitigation is disabled. So if you have a core which
> is zen <4 and doesn't use SBPB then what happens?

I'm afraid I don't understand the question.  In that case write_ibpb()
uses IBPB and manually clears the RSB.

-- 
Josh