linux-kernel - Re: [PATCH 3/4] KVM: nVMX: accurate emulation of MSR_IA32_CR{0,4}

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Message-ID: <CALzav=dqx8soqvHNEFD-M4+Fnq8oZoJ+gYX2jR7F+3o-qeKu2A@mail.gmail.com>
Date:   Mon, 28 Nov 2016 13:51:46 -0800
From:   David Matlack <dmatlack@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jim Mattson <jmattson@...gle.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH 3/4] KVM: nVMX: accurate emulation of MSR_IA32_CR{0,4}_FIXED1

On Wed, Nov 23, 2016 at 3:28 PM, David Matlack <dmatlack@...gle.com> wrote:
> On Wed, Nov 23, 2016 at 2:11 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>> On 23/11/2016 23:07, David Matlack wrote:
>>> A downside of this scheme is we'd have to remember to update
>>> nested_vmx_cr4_fixed1_update() before giving VMs new CPUID bits. If we
>>> forget, a VM could end up with different values for CR{0,4}_FIXED0 for
>>> the same CPUID depending on which version of KVM you're running on.

I've realized my concern here doesn't make sense. Such a VM would
likely fail to enter VMX operation, or #GP (unexpectedly) at some
point later. Linux, for example, does not appear to consult
MSR_IA32_VMX_CR4_FIXED1 when determining which bits of CR4 it can use
(regardless of whether it is in VMX operation or not).

>>
>> If userspace doesn't obey KVM_GET_SUPPORTED_CPUID, all bets are off
>> anyway, so I don't think it's a big deal.  However, if you want to make
>> it generated by userspace, that would be fine as well!
>
> Ok let's generate them in userspace.

I'm more inclined to generate them in the kernel, given the above.