linux-kernel - Re: [PATCH v2] x86/entry/64: Do not use RDPID in paranoid entry to accomodate KVM

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Message-ID: <CAMzpN2jQofGQ18PsEobeqfGX6ux=xuun_SQZhY=E3n1pzvEoAQ@mail.gmail.com>
Date:   Tue, 25 Aug 2020 08:11:23 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Chang Seok Bae <chang.seok.bae@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sasha Levin <sashal@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v2] x86/entry/64: Do not use RDPID in paranoid entry to
 accomodate KVM

On Tue, Aug 25, 2020 at 6:44 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Fri, Aug 21 2020 at 11:35, Brian Gerst wrote:
> > On Fri, Aug 21, 2020 at 10:22 AM Sean Christopherson
> >> >  .macro GET_PERCPU_BASE reg:req
> >> > -     ALTERNATIVE \
> >> > -             "LOAD_CPU_AND_NODE_SEG_LIMIT \reg", \
> >> > -             "RDPID  \reg", \
> >>
> >> This was the only user of the RDPID macro, I assume we want to yank that out
> >> as well?
> >
> > No.  That one should be kept until the minimum binutils version is
> > raised to one that supports the RDPID opcode.
>
> The macro is unused and nothing in the kernel can use RDPID as we just
> established.

It is opencoded in vdso_read_cpunode(), but the RDPID macro can't be
used there.  So you are correct, it can be removed.

--
Brian Gerst