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

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Date:   Wed, 23 Nov 2016 23:11:52 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Matlack <dmatlack@...gle.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 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.

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!  That would
simply entail removing this patch, wouldn't it?

Paolo

> Hm, now I'm thinking you were right in the beginning. Userspace should
> generate CR{0,4}_FIXED1, not the kernel. And KVM should allow
> userspace to save/restore them.