On Wed, Feb 8, 2017 at 12:09 AM, Kyle Huey <me@xxxxxxxxxxxx> wrote: > Hardware support for faulting on the cpuid instruction is not required to > emulate it, because cpuid triggers a VM exit anyways. KVM handles the relevant > MSRs (MSR_PLATFORM_INFO and MSR_MISC_FEATURES_ENABLE) and upon a > cpuid-induced VM exit checks the cpuid faulting state and the CPL. > kvm_require_cpl is even kind enough to inject the GP fault for us. > > Signed-off-by: Kyle Huey <khuey@xxxxxxxxxxxx> > Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx> I have a couple of concerns about portions of this patch: 1) There are some backward compatibility issues: A) Suppose we have an old userspace that doesn't know it needs to zero MSR_PLATFORM_INFO to preserve existing behavior (to the extent possible). If a VM starts on a new kernel it could set the bit in MSR_MISC_FEATURES_ENABLES that enables CPUID faulting. On live-migration to an old kernel, that bit would be lost. B) With either an old userspace or a new userspace, as a VM migrates between old and new kernels, the behavior of RDMSR with ECX set to either MSR_PLATFORM_INFO or MSR_MISC_FEATURES_ENABLES will vary depending on which kernel the VM is currently running on. Ideally, I think this new functionality should be guarded by a KVM capability that has to be enabled from userspace. 2) This doesn't really play well with volume 3 of the SDM, section 18.7.3, where Intel instructs developers to use MSR_PLATFORM_INFO[15:8] to determine the TSC frequency for a variety of microarchitectures. When reads of this MSR raised #GP, it was pretty clear that one couldn't get the TSC frequency that way, but I don't think many consumers would specifically check for a 0 in that field when the RDMSR succeeds. If a guest hypervisor used that value in the computation of the TSC scaling factor for a VMCS12, for example, it might be surprised to get a #DE.
