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From:   Nick Lowe <nick.lowe@gmail.com>
Date:   Fri, 16 Feb 2018 13:15:06 +0000
Message-ID: <CADSoG1uW5PuT9dr+w3npf6LiLvBL5=2x=uN98+JVNTR3KQbH5w@mail.gmail.com>
Subject: Re: [PATCH 4.9 43/92] x86/pti: Do not enable PTI on CPUs which are
 not vulnerable to Meltdown
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Arjan van de Ven <arjan@linux.intel.com>,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        David Woodhouse <dwmw@amazon.co.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@suse.de>,
        Dave Hansen <dave.hansen@intel.com>,
        gnomes@lxorguk.ukuu.org.uk, ak@linux.intel.com,
        ashok.raj@intel.com, karahmed@amazon.de,
        torvalds@linux-foundation.org, peterz@infradead.org,
        Borislav Petkov <bp@alien8.de>, pbonzini@redhat.com,
        tim.c.chen@linux.intel.com
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Hi

I do not have a tested patch, but I expect the change would be something like:

skip:
-       if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD)
+       if (!static_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
               goto disable;

Cheers,

Nick


On Tue, Feb 13, 2018 at 4:32 PM, Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
> On Tue, Feb 13, 2018 at 03:27:47PM +0000, Nick Lowe wrote:
>> Hi Arjan and Greg,
>>
>> Sorry if I am not being clear enough.
>>
>> My point is that there is a check for X86_VENDOR_AMD now in two places.
>>
>> It is still hardcoded for the auto boot option which I think should
>> not be there. The patch on that basis looked incomplete to me.
>>
>> Put another way, there is no effect to the auto option where the
>> contents of cpu_no_meltdown[] are changed and
>> cpu_vulnerable_to_meltdown returns differently.
>>
>> The auto option does not make use of a determination of the
>> X86_BUG_CPU_MELTDOWN state.
>>
>> This seems wrong to me. It does not seem correct to me for the auto
>> option to have this duplication with a check for just X86_VENDOR_AMD.
>
> Do you have a patch that reflects what you want to see changed here?
>
> And can you test it?  :)
>
> I don't have any AMD hardware, sorry.
>
> thanks,
>
> greg k-h