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

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Date:   Thu, 9 Mar 2017 17:29:42 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Brijesh Singh <brijesh.singh@....com>, simon.guinot@...uanux.org,
        linux-efi@...r.kernel.org, kvm@...r.kernel.org, rkrcmar@...hat.com,
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Subject: 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".
> 
> > +	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)
--