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Date: Thu, 11 Apr 2024 08:34:23 -0700
From: Josh Poimboeuf <jpoimboe@kernel.org>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Daniel Sneddon <daniel.sneddon@linux.intel.com>,
	Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Alexandre Chartre <alexandre.chartre@oracle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Sean Christopherson <seanjc@google.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Nikolay Borisov <nik.borisov@suse.com>,
	KP Singh <kpsingh@kernel.org>, Waiman Long <longman@redhat.com>,
	Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH 3/7] x86/bugs: Fix BHI handling of RRSBA
Message-ID: <20240411153423.6oolchbhjvnk2jm3@treble>
References: <cover.1712813475.git.jpoimboe@kernel.org>
 <6f56f13da34a0834b69163467449be7f58f253dc.1712813475.git.jpoimboe@kernel.org>
 <997d9bf9-2a8b-452f-b458-2fa15f92918e@citrix.com>
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In-Reply-To: <997d9bf9-2a8b-452f-b458-2fa15f92918e@citrix.com>

On Thu, Apr 11, 2024 at 11:02:42AM +0100, Andrew Cooper wrote:
> >  /* Disable in-kernel use of non-RSB RET predictors */
> >  static void __init spec_ctrl_disable_kernel_rrsba(void)
> >  {
> > -	u64 ia32_cap;
> > +	if (rrsba_disabled)
> > +		return;
> > +
> > +	if (!(ia32_cap & ARCH_CAP_RRSBA)) {
> > +		rrsba_disabled = true;
> > +		return;
> > +	}
> 
> You'll take this path if you have out-of-date microcode.
> 
> RRSBA is only enumerated from September last year, IIRC.  (Definitely
> from this point on some CPUs.)
> 
> When RRSBA was introduced, I was under the (false) impression that all
> eIBRS systems suffered RRSBA, but it turns out that select parts
> (ICX,TGL,RKL) are non-RRSBA despite not having RRSBA_CTRL.

Hm, so the original code here had this problem too, right?

	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE) &&
	    !(x86_read_arch_cap_msr() & ARCH_CAP_RRSBA))
		return;

At this point I'm having a hard time caring about 7 months out-of-date
microcode, but is there a reasonable way to check for that?

-- 
Josh