Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:7 as permitted sender) client-ip=2620:137:e000::3:7;
MIME-Version: 1.0
References: <20231004002038.907778-1-jmattson@google.com> <01009a2a-929e-ce16-6f44-1d314e6bcba5@intel.com>
 <CALMp9eR+Qudg++J_dmY_SGbM_kr=GQcRRcjuUxtm9rfaC_qeXQ@mail.gmail.com> <dfdba4e2-371c-db18-6989-541f802a0783@intel.com>
In-Reply-To: <dfdba4e2-371c-db18-6989-541f802a0783@intel.com>
From:   Jim Mattson <jmattson@google.com>
Date:   Tue, 3 Oct 2023 21:24:12 -0700
Message-ID: <CALMp9eRGrwYzmDxJmhNi1v-1dDgjoC0PUc-RfGiw6JVcMBnpqg@mail.gmail.com>
Subject: Re: [PATCH] x86: KVM: Add feature flag for AMD's FsGsKernelGsBaseNonSerializing
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Jiaxi Chen <jiaxi.chen@linux.intel.com>,
        Kim Phillips <kim.phillips@amd.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Tue, Oct 3, 2023 at 8:27=E2=80=AFPM Dave Hansen <dave.hansen@intel.com> =
wrote:
>
> On 10/3/23 19:44, Jim Mattson wrote:
> > I'm a little surprised at the pushback, TBH. Are you implying that
> > there is some advantage to *not* passing this bit through?
>
> I'm not really trying to push back.  I'm honestly just curious.  Linux
> obviously doesn't cat about the bit.  So is this for some future Linux
> or some other OS?

It's not for any particular guest OS. It's just for correctness of the
virtual CPU. Pedantically, hardware that enumerates this bit cannot
run a guest that doesn't. Pragmatically, it almost certainly doesn't
matter. Getting it right is trivial and has no impact on performance
or code size, so why not just do it?