linux-kernel - Re: [PATCH] x86/speculation: Mitigate eIBRS PBRSB predictions with WRMSR

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Date:   Thu, 6 Oct 2022 13:27:03 -0700
From:   "pawan.kumar.gupta@...ux.intel.com" 
        <pawan.kumar.gupta@...ux.intel.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     'Jim Mattson' <jmattson@...gle.com>,
        Suraj Jitindar Singh <surajjs@...zon.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "sjitindarsingh@...il.com" <sjitindarsingh@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>, "bp@...e.de" <bp@...e.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "jpoimboe@...nel.org" <jpoimboe@...nel.org>,
        "daniel.sneddon@...ux.intel.com" <daniel.sneddon@...ux.intel.com>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] x86/speculation: Mitigate eIBRS PBRSB predictions with
 WRMSR

On Thu, Oct 06, 2022 at 08:25:15AM +0000, David Laight wrote:
>From: Jim Mattson
>> Sent: 05 October 2022 23:29
>>
>> On Wed, Oct 5, 2022 at 3:03 PM Suraj Jitindar Singh <surajjs@...zon.com> wrote:
>> >
>> > tl;dr: The existing mitigation for eIBRS PBRSB predictions uses an INT3 to
>> > ensure a call instruction retires before a following unbalanced RET. Replace
>> > this with a WRMSR serialising instruction which has a lower performance
>> > penalty.
>>
>> The INT3 is only on a speculative path and should not impact performance.
>
>Doesn't that depend on how quickly the cpu can abort the
>decode and execution of the INT3 instruction?
>INT3 is bound to generate a lot of uops and/or be microcoded.
>
>Old cpu couldn't abort fpu instructions.
>IIRC the Intel performance guide even suggested not interleaving
>code and data because the data might get speculatively executed
>and take a long time to abort.
>
>I actually wonder whether 'JMPS .' (eb fe) shouldn't be used
>instead of INT3 (cc) because it is fast to decode and execute.
>But I'm no expect here.

I have been told that INT3 is better in this case because 'JMP .' would
waste CPU resources.