Re: [RFC] x86 Host CPU features detection by MSRs

Hello, This mail is a Request for Comment. On recent Intel CPUs, some of the CPU features (mostly vmx-* subfeatures) are listed and controlled via the MSRs (Model Specific Registers) instead of the traditional CPUID instruction method. Right now, libvirt reads the MSR's values via /dev/cpu/*/cpu populated by the msr kernel module. src/cpu/cpu_x86.c: ... /* This is best effort since there might be no way to read the MSR * when we are not running as root. */ for (i = 0; i < nmsrs; i++) { if (virHostCPUGetMSR(msrs[i], &msr) == 0) { virCPUx86DataItem item = { .type = VIR_CPU_X86_DATA_MSR, .data.msr = { ... There are 2 potential issues: 1) As stated in the source code comment above, MSR values read might fail when libvirt is not run as root. 2) MSR values read might fail if the MSR kernel module is not loaded, this is still the case in some of the Linux distros.

Andrea Bolognani's proposal of an approach that could work: 1. try using the unprivileged KVM API is available (or maybe the necessary information is exposed via QMP too?) like how QEMU does; 2. if that fails, try using the privileged /dev API;

3. if that fails too, load the msr module and try again;