Re: [PATCH RFC 01/39] KVM: x86: fix Xen hypercall page msr handling

[Sean Christopherson]

On Wed, Feb 20, 2019 at 08:15:31PM +0000, Joao Martins wrote:
> Xen usually places its MSR at 0x4000000 or 0x4000200 depending on
> whether it is running in viridian mode or not. Note that this is not
> ABI guaranteed, so it is possible for Xen to advertise the MSR some
> place else.
> 
> Given the way xen_hvm_config() is handled, if the former address is
> selected, this will conflict with HyperV's MSR
> (HV_X64_MSR_GUEST_OS_ID) which uses the same address.

Unconditionally servicing Hyper-V and KVM MSRs seems wrong, i.e. KVM
should only expose MSRs specific to a hypervisor if userspace has
configured CPUID to advertise support for said hypervisor.  If we do the
same thing for Xen, then the common MSR code looks like:

	if (kvm_advertise_kvm()) {
		if (<handle kvm msr>)
			return ...;
	} else if (kvm_advertise_hyperv()) {
		if (<handle hyperv msr>)
			return ...;
	} else if (kvm_advertise_xen()) {
		if (<handle xen msrs>)
			return ...;
	}

	<fall through to main switch statement>

Obviously assumes KVM only advertises itself as one hypervisor, and so
the ordering is arbitrary.

> Given that the MSR location is arbitrary, move the xen_hvm_config()
> handling to the top of kvm_set_msr_common() before falling through.
> Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 65e4559eef2f..47360a4b0d42 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2429,6 +2429,9 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  	u32 msr = msr_info->index;
>  	u64 data = msr_info->data;
>  
> +	if (msr && (msr == vcpu->kvm->arch.xen_hvm_config.msr))
> +		return xen_hvm_config(vcpu, data);
> +
>  	switch (msr) {
>  	case MSR_AMD64_NB_CFG:
>  	case MSR_IA32_UCODE_WRITE:
> @@ -2644,8 +2647,6 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		vcpu->arch.msr_misc_features_enables = data;
>  		break;
>  	default:
> -		if (msr && (msr == vcpu->kvm->arch.xen_hvm_config.msr))
> -			return xen_hvm_config(vcpu, data);
>  		if (kvm_pmu_is_valid_msr(vcpu, msr))
>  			return kvm_pmu_set_msr(vcpu, msr_info);
>  		if (!ignore_msrs) {
> -- 
> 2.11.0
>