Re: [RFC PATCH v3 01/27] x86/cpufeatures: Add SGX1 and SGX2 sub-features

Kai Huang
2021-01-27T02:49:13Z

On Tue, 2021-01-26 at 07:34 -0800, Dave Hansen wrote:
> On 1/26/21 1:30 AM, Kai Huang wrote:
> > From: Sean Christopherson <seanjc@google.com>
> >
> > Add SGX1 and SGX2 feature flags, via CPUID.0x12.0x0.EAX, as scattered
> > features, since adding a new leaf for only two bits would be wasteful.
> > As part of virtualizing SGX, KVM will expose the SGX CPUID leafs to its
> > guest, and to do so correctly needs to query hardware and kernel support
> > for SGX1 and SGX2.
>
> It's also not _just_ exposing the CPUID leaves. There are some checks
> here when KVM is emulating some SGX instructions too, right?

I would say trapping instead of emulating, but yes KVM will do more. However those
are quite details, and I don't think we should put lots of details here. Or perhaps
we can use 'for instance' as brief description:

As part of virtualizing SGX, KVM will need to use the two flags, for instance, to
expose them to guest.

?

>
> > diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> > index 84b887825f12..18b2d0c8bbbe 100644
> > --- a/arch/x86/include/asm/cpufeatures.h
> > +++ b/arch/x86/include/asm/cpufeatures.h
> > @@ -292,6 +292,8 @@
> >  #define X86_FEATURE_FENCE_SWAPGS_KERNEL (11*32+ 5) /* "" LFENCE in kernel entry SWAPGS path */
> >  #define X86_FEATURE_SPLIT_LOCK_DETECT (11*32+ 6) /* #AC for split lock */
> >  #define X86_FEATURE_PER_THREAD_MBA (11*32+ 7) /* "" Per-thread Memory Bandwidth Allocation */
> > +#define X86_FEATURE_SGX1 (11*32+ 8) /* Software Guard Extensions sub-feature SGX1 */
> > +#define X86_FEATURE_SGX2 (11*32+ 9) /* Software Guard Extensions sub-feature SGX2 */
>
> FWIW, I'm not sure how valuable it is to spell the SGX acronym out three
> times. Can't we use those bytes to put something more useful in that
> comment?

I think we can remove comment for SGX1, since it is basically SGX.

For SGX2, how about below?

/* SGX Enclave Dynamic Memory Management */