On Wed, Mar 4, 2020 at 5:34 PM Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote: > > Rework the masking in the out-of-range CPUID logic to handle the > Hypervisor sub-classes, as well as the Centaur class if the guest > virtual CPU vendor is Centaur. > > Masking against 0x80000000 only handles basic and extended leafs, which > results in Hypervisor range checks being performed against the basic > CPUID class, and Centuar range checks being performed against the > Extended class. E.g. if CPUID.0x40000000.EAX returns 0x4000000A and > there is no entry for CPUID.0x40000006, then function 0x40000006 would > be incorrectly reported as out of bounds. > > While there is no official definition of what constitutes a class, the > convention established for Hypervisor classes effectively uses bits 31:8 > as the mask by virtue of checking for different bases in increments of > 0x100, e.g. KVM advertises its CPUID functions starting at 0x40000100 > when HyperV features are advertised at the default base of 0x40000000. > > The bad range check doesn't cause functional problems for any known VMM > because out-of-range semantics only come into play if the exact entry > isn't found, and VMMs either support a very limited Hypervisor range, > e.g. the official KVM range is 0x40000000-0x40000001 (effectively no > room for undefined leafs) or explicitly defines gaps to be zero, e.g. > Qemu explicitly creates zeroed entries up to the Cenatur and Hypervisor > limits (the latter comes into play when providing HyperV features). > > The bad behavior can be visually confirmed by dumping CPUID output in > the guest when running Qemu with a stable TSC, as Qemu extends the limit > of range 0x40000000 to 0x40000010 to advertise VMware's cpuid_freq, > without defining zeroed entries for 0x40000002 - 0x4000000f. > > Note, documentation of Centaur/VIA CPUs is hard to come by. Designating > 0xc0000000 - 0xcfffffff as the Centaur class is a best guess as to the > behavior of a real Centaur/VIA CPU. Don't forget Transmeta's CPUID range at 0x80860000 through 0x8086FFFF!
