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

On Thu, Mar 09, 2017 at 05:13:33PM +0100, Paolo Bonzini wrote:

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.

Ah, good point, I already do that in the microcode loader :)

        /*
         * CPUID(1).ECX[31]: reserved for hypervisor use. This is still not
         * completely accurate as xen pv guests don't see that CPUID bit set but
         * that's good enough as they don't land on the BSP path anyway.
         */
        if (native_cpuid_ecx(1) & BIT(31))
                return *res;

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".

quoted

+	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))

Right, so this is testing CPUID_0x8000001f_ECX(0)[0], SME. Why not
simply set that bit for the guest too, in kvm?

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
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