linux-kernel - Re: [PATCH v6 3/7] x86/cpu, kvm: Move the LFENCE_RDTSC / LFENCE always serializing feature

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Message-ID: <cc344449-5f0b-51c7-bae8-6deb9be83702@intel.com>
Date:   Mon, 16 Jan 2023 13:15:29 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Borislav Petkov <bp@...en8.de>,
        Kim Phillips <kim.phillips@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     x86@...nel.org, Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Jonathan Corbet <corbet@....net>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tony Luck <tony.luck@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Alexey Kardashevskiy <aik@....com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 3/7] x86/cpu, kvm: Move the LFENCE_RDTSC / LFENCE
 always serializing feature

On 1/16/23 10:13, Borislav Petkov wrote:
>>  /* AMD-defined Extended Feature 2 EAX, CPUID level 0x80000021 (EAX), word 20 */
>>  #define X86_FEATURE_NO_NESTED_DATA_BP	(20*32+ 0) /* "" AMD No Nested Data Breakpoints */
>> +#define X86_FEATURE_LFENCE_RDTSC	(20*32+ 2) /* "" LFENCE always serializing / synchronizes RDTSC */
> Hmm, a synthetic bit which gets replaced with a vendor one and then the other
> vendors set it too. I don't see why that cannot work but we probably should be
> careful here.
> 
> dhansen, am I missing an angle?

I don't think so.

I'd be surprised if we don't have a _few_ other cases like this around,
but nothing is coming to mind.  Either way, it doesn't seem problematic.