Re: [PATCH] x86/retpoline: Fill RSB on context switch for affected CPUs

From: David Woodhouse <dwmw2@infradead.org>
To: Andi Kleen <ak@linux.intel.com>
Cc: Paul Turner <pjt@google.com>, LKML <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linux-foundation.org>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	tglx@linutronix.de, Kees Cook <keescook@google.com>,
	Rik van Riel <riel@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Jiri Kosina <jikos@kernel.org>,
	gnomes@lxorguk.ukuu.org.uk, x86@kernel.org,
	thomas.lendacky@amd.com, Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH] x86/retpoline: Fill RSB on context switch for affected CPUs
Date: Fri, 12 Jan 2018 18:23:07 +0000	[thread overview]
Message-ID: <1515781387.22302.472.camel@infradead.org> (raw)
In-Reply-To: <20180112180224.GP6718@tassilo.jf.intel.com>

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On Fri, 2018-01-12 at 10:02 -0800, Andi Kleen wrote:
> > +     if ((!boot_cpu_has(X86_FEATURE_PTI) &&
> > +          !boot_cpu_has(X86_FEATURE_SMEP)) || is_skylake_era()) {
> > +             setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
> > +             pr_info("Filling RSB on context switch\n");
> 
> We need to do more things for Skylake (like idle and interrupt fill
> and possibly deep call cahin), so I don't think it makes sense to
> 
> - have an individual flag for each of these. It can be just a single
> flag that enables all of this for Skylake
> 
> - print something for each of them. that will just be very noisy
> without any useful benefit to the user.

I still think we are better off using IBRS by default on Skylake.

This patch wasn't really for Skylake; the real use case was for AMD
CPUs (!PTI) without SMEP. Since it happens to needed on Skylake too we
might as well enable it there... but that doesn't mean I was planning
to do all the other horrible crap we need for Skylake.

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