linux-kernel - Re: [PATCH v2 1/4] x86/cpufeatures: Add enumeration for SERIALIZE instruction

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Date:   Tue, 18 Aug 2020 09:14:58 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Cathy Zhang <cathy.zhang@...el.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kyung Min Park <kyung.min.park@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>, fenghua.yu@...el.com
Subject: Re: [PATCH v2 1/4] x86/cpufeatures: Add enumeration for SERIALIZE
 instruction

On 23/07/20 01:02, Thomas Gleixner wrote:
>> I am cc'ing Fenghua, who has volunteered to work on this. Addind support
>> for SERIALIZE in sync_core() should not block merging these patches,
>> correct?
> Come on. We are not serving KVM first before making this usable on bare
> metal.

This in the end was merged in 5.9, but: why not?  It is just an
instruction with no other support code needed in the kernel (or KVM for
that matter except for marking the CPUID bit as supported).  It is
common to run hosts with an older kernel than the guests, and by this
line of reasoning, we should not even have enabled support for FSGSBASE
in KVM.

Paolo