linux-kernel - Re: [PATCH v5 02/12] x86/retpoline: Add initial retpoline support

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Date:   Sat, 6 Jan 2018 19:44:38 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     "Woodhouse, David" <dwmw@...zon.co.uk>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andi Kleen <ak@...ux.intel.com>
Cc:     Paul Turner <pjt@...gle.com>, LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, tglx@...utronix.de,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v5 02/12] x86/retpoline: Add initial retpoline support

On 1/6/2018 3:21 PM, Woodhouse, David wrote:
> On Sat, 2018-01-06 at 21:16 +0000, Andrew Cooper wrote:
>> On 06/01/18 11:49, David Woodhouse wrote:
>>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>>> index 372ba3f..40e6e54 100644
>>> --- a/arch/x86/kernel/cpu/common.c
>>> +++ b/arch/x86/kernel/cpu/common.c
>>> @@ -904,6 +904,11 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
>>>   
>>>        setup_force_cpu_bug(X86_BUG_SPECTRE_V1);
>>>        setup_force_cpu_bug(X86_BUG_SPECTRE_V2);
>>> +#ifdef CONFIG_RETPOLINE
>>> +     setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
>>> +     if (c->x86_vendor == X86_VENDOR_AMD)
>>> +             setup_force_cpu_cap(X86_FEATURE_RETPOLINE_AMD);
>>
>> This isn't safe.  It needs to be dependant on finding that LFENCEs are
>> actually dispatch serialising.
>>
>> In particular, when virtualised, you'll most likely be saddled with the
>> hypervisors choice of setting, in which case you need to use retpoline
>> as a fallback.
> 
> Thanks. I was about to rebase on top of tip/x86/pti which has Tom's
> patches to make lfence serialising — which seem to say that if the MSR
> isn't available, it *will* be serialising.
> 
> I think I'll just refrain from setting X86_FEATURE_RETPOLINE_AMD for
> now, and let Tom turn that on in his own time.

I can do that.  I'll move it to arch/x86/kernel/cpu/amd.c to just after
the line that sets the MSR bit making lfence serializing.  I'll submit
that once your patches are pulled in (or at least the feature bits).

Thanks,
Tom

>