linux-kernel - Re: [patch v3 2/7] x86/smp: Dont access non-existing CPUID leaf

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Message-ID: <87v8fjpe5m.ffs@tglx>
Date:   Mon, 19 Jun 2023 19:15:01 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Limonciello, Mario" <mario.limonciello@....com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Tom Lendacky <thomas.lendacky@....com>,
        Tony Battersby <tonyb@...ernetics.com>,
        Ashok Raj <ashok.raj@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Arjan van de Veen <arjan@...ux.intel.com>,
        Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [patch v3 2/7] x86/smp: Dont access non-existing CPUID leaf

On Mon, Jun 19 2023 at 12:02, Limonciello, Mario wrote:
> On 6/15/2023 3:33 PM, Thomas Gleixner wrote:
>> From: Tony Battersby <tonyb@...ernetics.com>
>>
>> stop_this_cpu() tests CPUID leaf 0x8000001f::EAX unconditionally. CPUs
>> return the content of the highest supported leaf when a non-existing leaf
>> is read. So the result of the test is lottery except on AMD CPUs which
>> support that leaf.
>>
>> While harmless it's incorrect and causes the conditional wbinvd() to be
>> issued where not required.
>>
>> Check whether the leaf is supported before reading it.
>>
>> [ tglx: Adjusted changelog ]
>>
>> Fixes: 08f253ec3767 ("x86/cpu: Clear SME feature flag when not in use")
>
> Thanks for this fix.
> This particular patch should probably also CC to stable.

It's pretty much all stable material.