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

[Daniel P. Berrangé]

On Fri, Apr 12, 2019 at 11:47:15AM +0200, Jiri Denemark wrote:
> 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.

Hmm, do we need to get /dev/kvm fixed in this regard ?  It seems bad that
libvirt will be relying on different information about MSRs when running
as root vs non-root. Could like to even more painful bugs to triage.

Regards,
Daniel
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