Date:   Wed, 22 Feb 2023 18:48:41 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     KP Singh <kpsingh@kernel.org>
Cc:     linux-kernel@vger.kernel.org, pjt@google.com, evn@google.com,
        jpoimboe@kernel.org, tglx@linutronix.de, x86@kernel.org,
        hpa@zytor.com, peterz@infradead.org,
        pawan.kumar.gupta@linux.intel.com, kim.phillips@amd.com,
        alexandre.chartre@oracle.com, daniel.sneddon@linux.intel.com,
        corbet@lwn.net, bp@suse.de, linyujun809@huawei.com,
        jmattson@google.com,
        =?utf-8?B?Sm9zw6k=?= Oliveira <joseloliveira11@gmail.com>,
        Rodrigo Branco <rodrigo@kernelhacking.com>,
        Alexandra Sandulescu <aesa@google.com>, stable@vger.kernel.org
Subject: Re: [PATCH v2 1/2] x86/speculation: Allow enabling STIBP with legacy
 IBRS
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On Wed, Feb 22, 2023 at 09:16:21AM -0800, KP Singh wrote:
> Thanks for iterating. I think your commit description and rewrite
> omits a few key subtleties which I have tried to reinforce in both the
> commit log and the comments.
> 
> Q: What does STIBP have to do with IBRS?
> A: Setting the IBRS bit implicitly enables STIBP / some form of cross
> thread protection.

That belongs in the docs, if you want to explain this properly.

> Q: Why does it work with eIBRS?
> A: Because we set the IBRS bit once and leave it set when using eIBRS

Also docs.

> I think this subtlety should be reinforced in the commit description
> and code comments so that we don't get it wrong again. Your commit
> does answer this one (thanks!)

Commit messages are fine when explaining *why* a change is being done.
What is even finer is when you put a lenghtier explanation in our
documentation so that people can actually find it. Finding text in
commit messages is harder...

> Q: Why does it not work with the way the kernel currently implements
> legacy IBRS?
> A: Because the kernel clears the bit on returning to user space.

From the commit message:

    However, on return to userspace, the IBRS bit is cleared for performance
    reasons. That leaves userspace threads vulnerable to cross-thread
    predictions influence against which STIBP protects.

> The reason why I refactored this into a separate helper was to
> document the subtleties I mentioned above and anchor them to one place
> as the function is used in 2 places. But this is a maintainer's
> choice, so it's your call :)

The less code gets added in that thing, the better. Not yet another
helper pls.
 
> I do agree with Pawan that it's worth adding a pr_info about what the
> kernel is doing about STIBP.

STIBP status gets dumped through stibp_state().

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette