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Message-ID: <1517433465.18619.209.camel@amazon.co.uk>
Subject: Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to
 MSR_IA32_SPEC_CTRL
To:     Jim Mattson <jmattson@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>
CC:     KarimAllah Ahmed <karahmed@amazon.com>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        "Andi Kleen" <ak@linux.intel.com>,
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        "Linus Torvalds" <torvalds@linux-foundation.org>,
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        Thomas Gleixner <tglx@linutronix.de>,
        "Dan Williams" <dan.j.williams@intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Ashok Raj <ashok.raj@intel.com>
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Content-Type: multipart/signed; micalg=sha-256; protocol="application/x-pkcs7-signature"; boundary="=-gJf04PP6HFULl0xoPmKi"
Date:   Wed, 31 Jan 2018 21:17:45 +0000
MIME-Version: 1.0
From:   "Woodhouse, David" <dwmw@amazon.co.uk>
Sender: linux-kernel-owner@vger.kernel.org
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Content-Type: text/plain; charset="UTF-8"
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On Wed, 2018-01-31 at 13:05 -0800, Jim Mattson wrote:
> On Wed, Jan 31, 2018 at 1:00 PM, Paolo Bonzini <pbonzini@redhat.com> wrot=
e:
>=20
> > Yes, but how would moving the field into struct loaded_vmcs do anything=
?
> >=C2=A0 Only vmon/vmoff would change anything in vmx->nested.vmcs02.
>=20
> My suggestion was that nested_vmx_merge_msr_bitmap should set the
> vmcs02 version of save_spec_ctrl_on_exit based on the calculated value
> of the write permission bit for IA32_SPEC_CTRL in the vmcs02 MSR
> permission bitmap.
>=20
> > Even then, L1 vmexits will also be penalized because L1 has probably
> > done an RDMSR/WRMSR on L2->L1 vmexit.=C2=A0 So I don't think it's an is=
sue?
>=20
> Yes, it sucks to be L1 in this situation.

Well... we *could* clear the save_spec_ctrl_on_exit flag and intercept
the MSR again, any time that the actual value of spec_ctrl is zero.=C2=A0

I don't think we'd want to do that too aggressively, but there might be
something we could do there.
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