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Subject: Re: [PATCH 23/35] x86/speculation: Add basic speculation control code
To: Josh Poimboeuf <jpoimboe@redhat.com>,
 Peter Zijlstra <peterz@infradead.org>
References: <20180118134800.711245485@infradead.org>
 <20180118140152.830682032@infradead.org>
 <20180118163745.t5nmwdr53wjsl7o5@treble>
Cc: David Woodhouse <dwmw2@infradead.org>,
 Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
 Ashok Raj <ashok.raj@intel.com>, Tim Chen <tim.c.chen@linux.intel.com>,
 Andy Lutomirski <luto@kernel.org>,
 Linus Torvalds <torvalds@linux-foundation.org>,
 Greg KH <gregkh@linuxfoundation.org>, Andrea Arcangeli
 <aarcange@redhat.com>, Andi Kleen <ak@linux.intel.com>,
 Arjan Van De Ven <arjan.van.de.ven@intel.com>,
 Dan Williams <dan.j.williams@intel.com>, Paolo Bonzini
 <pbonzini@redhat.com>, Jun Nakajima <jun.nakajima@intel.com>,
 Asit Mallick <asit.k.mallick@intel.com>, Jason Baron <jbaron@akamai.com>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <73a5735a-6a5b-0e0f-1f0b-e7cd955880d2@intel.com>
Date: Thu, 18 Jan 2018 09:08:02 -0800
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On 01/18/2018 08:37 AM, Josh Poimboeuf wrote:
>>
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -3932,6 +3932,7 @@
>>  			retpoline	  - replace indirect branches
>>  			retpoline,generic - google's original retpoline
>>  			retpoline,amd     - AMD-specific minimal thunk
>> +			ibrs		  - Intel: Indirect Branch Restricted Speculation
> Are there plans to add spectre_v2=ibrs_always to prevent SMT-based
> attacks?

What does "ibrs_always" mean to you?

There is a second bit in the MSR (STIBP) that is intended to keep
hyperthreads from influencing each-other.  That is behavior is implicit
when IBRS is enabled.

I think ibrs_always *should* probably be kept to refer to the future
CPUs that can safely leave IBRS enabled all the time.