Re: [RFC PATCH v2 12/32] x86: Add early boot support when running with SEV active

[Paolo Bonzini]

On 09/03/2017 15:07, Borislav Petkov wrote:
> +	/* Check if running under a hypervisor */
> +	eax = 0x40000000;
> +	ecx = 0;
> +	native_cpuid(&eax, &ebx, &ecx, &edx);

This is not how you check if running under a hypervisor; you should
check the HYPERVISOR bit, i.e. bit 31 of cpuid(1).ecx.  This in turn
tells you if leaf 0x40000000 is valid.

That said, the main issue with this function is that it hardcodes the
behavior for KVM.  It is possible that another hypervisor defines its
0x40000001 leaf in such a way that KVM_FEATURE_SEV has a different meaning.

Instead, AMD should define a "well-known" bit in its own space (i.e.
0x800000xx) that is only used by hypervisors that support SEV.  This is
similar to how Intel defined one bit in leaf 1 to say "is leaf
0x40000000 valid".

Thanks,

Paolo

> +	if (eax > 0x40000000) {
> +		eax = 0x40000001;
> +		ecx = 0;
> +		native_cpuid(&eax, &ebx, &ecx, &edx);
> +		if (!(eax & BIT(KVM_FEATURE_SEV)))
> +			goto out;
> +
> +		eax = 0x8000001f;
> +		ecx = 0;
> +		native_cpuid(&eax, &ebx, &ecx, &edx);
> +		if (!(eax & 1))
> +			goto out;
> +
> +		sme_me_mask = 1UL << (ebx & 0x3f);
> +		sev_enabled = 1;
> +
> +		goto out;
> +	}
> +