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From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: David Hildenbrand <david@redhat.com>,
        Wanpeng Li <kernellwp@gmail.com>, kvm <kvm@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
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        "K. Y. Srinivasan" <kys@microsoft.com>,
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        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@microsoft.com>,
        Mohammed Gamal <mmorsy@redhat.com>,
        Cathy Avery <cavery@redhat.com>, Bandan Das <bsd@redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC 0/6] Enlightened VMCS support for KVM on Hyper-V
References: <20180115173105.31845-1-vkuznets@redhat.com>
        <CANRm+Cza82KuvEQQi=2Zio2u0EHm1kfPfr=aOhoLdra+fpB2Wg@mail.gmail.com>
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        <fce01165-912c-a592-2f10-71041ab13b29@redhat.com>
Date: Tue, 16 Jan 2018 15:43:56 +0100
In-Reply-To: <fce01165-912c-a592-2f10-71041ab13b29@redhat.com> (Paolo
        Bonzini's message of "Tue, 16 Jan 2018 15:27:33 +0100")
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Paolo Bonzini <pbonzini@redhat.com> writes:

> On 16/01/2018 14:58, Vitaly Kuznetsov wrote:
>>> Haven't looked into the details, but we have to watch out for other
>>> VCPUs trying to modify that vmcs12.
>>>
>>> Basically because other VCPUs could try to modify values in vmcs12 while
>>> we are currently building vmcs02. Nasty races could result in us copying
>>> stuff (probably unchecked) into vmcs02 and therefore running something
>>> that was not intended.
>>>
>> I don't think we share VMCS among vCPUs, do we?
>
> VMCS is just memory, so who knows what a malicious L1 guest will do.
> But for vmread/vmwrite we can go through hypervisor memory, for
> enlightened VMCS we cannot.
>

True; not sure if Hyper-V actually copies the data to some internal
storage, probably it does. TLFS explicitly forbids making the same
enlightened VMCS active on several vCPUs simultaneously but again, this
is just memory...

-- 
  Vitaly