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.
