On 18 Jul 2006, at 11:14, Arjan van de Ven wrote: >> Allow subarchitectures to modify the CPUID instruction. This allows >> the subarch to provide a limited set of CPUID feature flags during CPU >> identification. Add a subarch implementation for Xen that traps to >> the >> hypervisor where unsupported feature flags can be hidden from guests. > > I'm wondering if this is entirely the wrong level of abstraction; to me > it feels the subarch shouldn't override the actual cpuid, but the cpu > feature flags that linux uses. That's a lot less messy: cpuid has many > many pieces of information which are near impossible to filter in > practice, however filtering the USAGE of it is trivial; linux basically > flattens the cpuid namespace into a simple bitmap of "what the kernel > can use". That is really what the subarch should filter/fixup, just > like > we do for cpu quirks etc etc. Maybe we should have that *as well*, but it makes sense to allow the hypervisor to apply a filter too. For example, whether it supports PSE, FXSAVE/FXRSTOR, etc. These are things the 'platform' is telling the OS -- not something the OS can filter for itself. To trap on CPUID invocations requires the guest to use a special code sequence for CPUID, since the instruction will never normally fault. Hence moving to mach-* to hide this detail. -- Keir
