linux-kernel - Re: [PATCH] x86/nospec: Unwreck the RSB stuffing

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Date:   Tue, 16 Aug 2022 15:34:50 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH] x86/nospec: Unwreck the RSB stuffing

On Tue, Aug 16, 2022 at 03:01:33PM +0200, Borislav Petkov wrote:
> On Tue, Aug 16, 2022 at 12:52:58PM +0000, Andrew Cooper wrote:
> > One minor point.  Stuff 32 slots.
> > 
> > There are Intel parts out in the world with RSBs larger than 32 entries,
> > and this loop does not fill the entire RSB on those.
> > 
> > This is why the 32-entry stuffing loop is explicitly not supported on
> > eIBRS hardware as a general mechanism.
> 
> I'm guessing there will be an Intel patch forthcoming, making that
> RSB_CLEAR_LOOPS more dynamic, based on the current model.

This is being discussed internally, but likely Enhanced IBRS parts don't
need RSB stuffing (except for the single entry stuffing for mitigating
PBRSB).