Re: [libvirt] [PATCH 34/36] vircpuhost: Add support for reading MSRs

On Thu, Apr 11, 2019 at 17:13:51 +0100, Daniel P. Berrangé wrote: > On Mon, Apr 08, 2019 at 10:42:38AM +0200, Jiri Denemark wrote: > > The new virHostCPUGetMSR internal API will try to read the MSR from > > /dev/cpu/0/msr and if it is not possible (the device does not exist or > > libvirt is running unprivileged), it will fallback to asking KVM for the > > MSR using KVM_GET_MSRS ioctl. > > Is there benefit to using /dev/cpu/0/msr, instead of always using > /dev/kvm ? KVM may report some bits in the IA32_ARCH_CAPABILITIES MSR even when the host does not set them or even support MSR at all. So for reporting host capabilities /dev/cpu/0/msr is IMHO better. We will still get the made up bits in the MSR via QEMU for host-model. For example, on my laptop which does not support IA32_ARCH_CAPABILITIES MSR at all I get 0x8 when asking KVM, while reading /dev/cpu/0/msr returns I/O error since the MSR does not exist.