Re: [PATCH v7 01/45] x86/compressed/64: detect/setup SEV/SME features earlier in boot

[Michael Roth]

On Fri, Nov 12, 2021 at 05:52:51PM +0100, Borislav Petkov wrote:
> On Wed, Nov 10, 2021 at 04:06:47PM -0600, Brijesh Singh wrote:
> > +void sev_enable(struct boot_params *bp)
> > +{
> > +	unsigned int eax, ebx, ecx, edx;
> > +
> > +	/* Check for the SME/SEV support leaf */
> > +	eax = 0x80000000;
> > +	ecx = 0;
> > +	native_cpuid(&eax, &ebx, &ecx, &edx);
> > +	if (eax < 0x8000001f)
> > +		return;
> > +
> > +	/*
> > +	 * Check for the SME/SEV feature:
> > +	 *   CPUID Fn8000_001F[EAX]
> > +	 *   - Bit 0 - Secure Memory Encryption support
> > +	 *   - Bit 1 - Secure Encrypted Virtualization support
> > +	 *   CPUID Fn8000_001F[EBX]
> > +	 *   - Bits 5:0 - Pagetable bit position used to indicate encryption
> > +	 */
> > +	eax = 0x8000001f;
> > +	ecx = 0;
> > +	native_cpuid(&eax, &ebx, &ecx, &edx);
> > +	/* Check whether SEV is supported */
> > +	if (!(eax & BIT(1)))
> > +		return;
> > +
> > +	/* Check the SEV MSR whether SEV or SME is enabled */
> > +	sev_status   = rd_sev_status_msr();
> > +
> > +	if (!(sev_status & MSR_AMD64_SEV_ENABLED))
> > +		error("SEV support indicated by CPUID, but not SEV status MSR.");
> 
> What is the practical purpose of this test?

In the current QEMU/KVM implementation the SEV* CPUID bits are only
exposed for SEV guests, so this was more of a sanity check on that. But
looking at things more closely: that's more of a VMM-specific behavior
and isn't necessarily an invalid guest configuration as far as the spec
is concerned, so I think this check should be dropped.

> 
> > +	sme_me_mask = 1UL << (ebx & 0x3f);
> 
> 	sme_me_mask = BIT_ULL(ebx & 0x3f);

Will do.

Thanks,

Mike

> 
> Thx.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
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