Re: [RFC PATCH 09/13] x86/cpufeature: Add detection of KVM XO

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Wed, 2019-10-30 at 07:55 -0700, Sean Christopherson wrote:
> On Tue, Oct 29, 2019 at 04:52:08PM -0700, Edgecombe, Rick P wrote:
> > On Tue, 2019-10-29 at 16:33 -0700, Kees Cook wrote:
> > > On Thu, Oct 03, 2019 at 02:23:56PM -0700, Rick Edgecombe wrote:
> > > > Add a new CPUID leaf to hold the contents of CPUID 0x40000030 EAX to
> > > > detect KVM defined generic VMM features.
> > > > 
> > > > The leaf was proposed to allow KVM to communicate features that are
> > > > defined by KVM, but available for any VMM to implement.
> 
> This doesn't necessarily work the way you intend, KVM's base CPUID isn't
> guaranteed to be 0x40000000.  E.g. KVM supports advertising itself as
> HyperV *and* KVM, in which case KVM's CPUID base will be 0x40000100.
> 
> I think you're better off just making this a standard KVM CPUID feature.
> If a different hypervisor wants to reuse guest support as is, it can
> advertise KVM support at a lower priority.
> 
Ok, I'm fine going with the simpler KVM CPUID bit. It's not like per-VMM CPUID
leaf meanings are a new scenario with this.

> Note, querying guest CPUID isn't straightforward in either case.  But,
> KVM doesn't support disabling its other CPUID-base paravirt features, e.g.
> KVM emulates the kvm_clock MSRs regardless of what userspace advertises to
> the guest.  Depending on what changes are required in KVM's MMU, this may
> also need to be a KVM-wide feature, i.e. controlled via a module param.
> > > > Add cpu_feature_enabled() support for features in this leaf (KVM XO),
> > > > and
> > > > a pgtable_kvmxo_enabled() helper similar to pgtable_l5_enabled() so that
> > > > pgtable_kvmxo_enabled() can be used in early code that includes
> > > > arch/x86/include/asm/sparsemem.h.
> > > > 
> > > > Lastly, in head64.c detect and this feature and perform necessary
> > > > adjustments to physical_mask.
> > > 
> > > Can this be exposed to /proc/cpuinfo so a guest userspace can determine
> > > if this feature is enabled?
> > > 
> > > -Kees
> > 
> > Is there a good place to expose the information that the PROT_EXEC and
> > !PROT_READ combo creates execute-only memory? This way apps can check one
> > place
> > for the support and not worry about the implementation whether it's this,
> > x86
> > pkeys, arm or other.
> 
> I don't think so?  Assuming there's no common method, it can be displayed
> in /proc/cpuinfo by adding a synthetic bit, e.g. in Linux-defined word 8
> (virtualization) instead of a dedicated word.  The bit can then be
> set if the features exists and is enabled (by the guest).
> 
> I'd also name the feature EXEC_ONLY.  XO is unnecessarily terse IMO, and
> including "KVM" in the name may be misconstrued as a host KVM feature and
> will be flat out wrong if hardware ever supports XO natively.

Ok, if there is no generic way I guess I'll do this.