linux-kernel - Re: [PATCH] x86/bugs: Switch to "auto" when "ibrs" selected on Enhanced IBRS parts

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Date:   Thu, 14 Jul 2022 11:42:29 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thadeu Lima de Souza Cascardo <cascardo@...onical.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        antonio.gomez.iglesias@...ux.intel.com
Subject: Re: [PATCH] x86/bugs: Switch to "auto" when "ibrs" selected on
 Enhanced IBRS parts

On Thu, Jul 14, 2022 at 10:38:14AM -0700, Josh Poimboeuf wrote:
>On Thu, Jul 14, 2022 at 07:03:32PM +0200, Peter Zijlstra wrote:
>> On Thu, Jul 14, 2022 at 09:01:06AM -0700, Josh Poimboeuf wrote:
>>
>> > > Yeah this; if the user asks for IBRS, we should give him IBRS. I hate
>> > > the 'I know better, let me change that for you' mentality.
>> >
>> > eIBRS CPUs don't even have legacy IBRS so I don't see how this is even
>> > possible.
>>
>> You can still WRMSR a lot on them. Might not make sense but it 'works'.
>
>Even in Intel documentation, eIBRS is often referred to as IBRS. It
>wouldn't be surprising for a user to consider spectre_v2=ibrs to mean
>"use eIBRS".
>
>I'm pretty sure there's nobody out there that wants spectre_v2=ibrs to
>mean "make it slower and possibly less secure because it's being used
>contrary to the spec".

Apart from testing, I don't see a reason for a user to deliberately
choose =ibrs on Enhanced IBRS parts. But, I am guessing most users would
just rely on "=auto" mode.

So honoring what the user asked and printing a warning may be fine. And
hope they would see the warning if they unintentionally chose "=ibrs" on
an eIBRS part.

Thanks,
Pawan