Re: [PATCH] KVM: VMX: expose the host's ARCH_CAPABILITIES MSR to userspace

[Paolo Bonzini]

On 02/03/2018 22:42, Radim KrÄmÃÅ wrote:
> Ok, sounds good.  I've deferred it to rc5 as I think we'll want to use
> this to replace the auto setting:  I would not bet that it is going to
> be safe to expose future bits, so having the userspace always sanitize
> the capabilities would be safer (and more in line with what we do with
> other MSRs).  i.e. this patch would also
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 051dab74e4e9..86ea4a83af1f 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5740,9 +5740,6 @@ static void vmx_vcpu_setup(struct vcpu_vmx *vmx)
>  		++vmx->nmsrs;
>  	}
>  
> -	if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))
> -		rdmsrl(MSR_IA32_ARCH_CAPABILITIES, vmx->arch_capabilities);
> -
>  	vm_exit_controls_init(vmx, vmcs_config.vmexit_ctrl);
>  
>  	/* 22.2.1, 20.8.1 */

I don't know. There are good reasons for both behaviors, and especially
the following two for _not_ removing the rdmsr:

1) so far you could just pass the result of KVM_GET_SUPPORTED_CPUID to
KVM_SET_CPUID2, and expect the result to be "as close as possible to the
host";

2) having different behavior for VMX and ARCH_CAPABILITIES MSRs would be
confusing.

I think I'm leaning more towards the following direction: whitelist
ARCH_CAPABILITIES, like we do for the AMD LFENCE MSR already, and
default the AMD LFENCE MSR to the host value.

Thanks,

Paolo