Date: Wed, 10 Jan 2018 09:50:16 +0100
From: Willy Tarreau <w@1wt.eu>
To: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Brian Gerst <brgerst@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Kees Cook <keescook@chromium.org>
Subject: Re: [RFC PATCH v2 3/6] x86/pti: add a per-cpu variable pti_disable
Message-ID: <20180110085016.GE14066@1wt.eu>
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On Wed, Jan 10, 2018 at 09:01:02AM +0100, Ingo Molnar wrote:
> 
> * Willy Tarreau <w@1wt.eu> wrote:
> 
> > [...] If we had "pit_enabled", something like this could be confusing because 
> > it's not obvious whether this pti_enabled *enforces* PTI or if its absence 
> > disables it :
> > 
> > 	cmpb $0, PER_CPU_VAR(pti_enabled)
> > 	jz .Lend\@
> 
> The natural sequence would be:
> 
> 	cmpb $1, PER_CPU_VAR(pti_enabled)
> 	jne .Lend\@
> 
> which is not confusing to me at all.

In fact I think I know now why it still poses me a problem : this
pti_enabled flag alone is not sufficient to enable PTI, it's just part
of the condition, as another part comes from the X86_FEATURE_PTI flag.
However, pti_disabled is sufficient to disable PTI so actually its
effect matches its name (note BTW that I called it "pti_disable" as a
verb indicating an action -- "I want to disable pti", and not as a past
form "pti is disabled").

Willy