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Subject: Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to
 MSR_IA32_SPEC_CTRL
To: Jim Mattson <jmattson@google.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>,
 Andrea Arcangeli <aarcange@redhat.com>,
 Linus Torvalds <torvalds@linux-foundation.org>,
 Tim Chen <tim.c.chen@linux.intel.com>, Thomas Gleixner <tglx@linutronix.de>,
 Dan Williams <dan.j.williams@intel.com>,
 Jun Nakajima <jun.nakajima@intel.com>, David Woodhouse <dwmw@amazon.co.uk>,
 Greg KH <gregkh@linuxfoundation.org>, Andy Lutomirski <luto@kernel.org>,
 Ashok Raj <ashok.raj@intel.com>
References: <1517427467-28567-1-git-send-email-karahmed@amazon.de>
 <1517427467-28567-5-git-send-email-karahmed@amazon.de>
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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <16d89e09-fbfc-4a08-00a6-40cfb4776018@redhat.com>
Date: Wed, 31 Jan 2018 16:00:43 -0500
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On 31/01/2018 15:54, Jim Mattson wrote:
> You seem to be making the assumption that there is one L2. What if
> there are 100 L2s, and only one has write-access to IA32_SPEC_CTRL? Or
> what if there once was such an L2, but it's been gone for months? The
> current mechanism penalizes *all* L2s if any L2, ever, has
> write-access to IA32_SPEC_CTRL.

Yes, but how would moving the field into struct loaded_vmcs do anything?
 Only vmon/vmoff would change anything in vmx->nested.vmcs02.

Even then, L1 vmexits will also be penalized because L1 has probably
done an RDMSR/WRMSR on L2->L1 vmexit.  So I don't think it's an issue?

Paolo