Re: [Qemu-devel] Question of emulation on MSR's in KVM-mode

[Paolo Bonzini]

Il 04/08/2014 10:37, Morty Andersen ha scritto:
> Hi
> 
> I'm working on an extension to QEMU (target i386). This involves adding
> new MSR's. I've got it working in non-KVM mode by adding these MSR's to
> the state and adding extra cases to helper_wrmsr(), helper_rdmsr(). The
> guest can now read/write these MSR's as expected. However, it fails when
> running in KVM-mode. Specifically, writing the MSR's causes GPF. Note
> that these MSR's are not natively supported by the host CPU. I don't
> know enough about Intel's VMX to tell if it is even reasonable to expect
> that this could work for a non-natively supported MSR. As far as I can
> read in the VMX documentation, the hypervisor can setup a bitmap of
> which MSR's should cause trap's to the hypervisor and which shouldn't. I
> guess it would be the KVM kernel module that does this based on input it
> receives from QEMU. But I haven't been able to find the part of QEMU
> that negotiates this. I guess the solution for me is to set the
> necessary bits to that access to the new MSR's causes traps. Next, I
> need to add/modify the trap handler so that it can handle the MSR's.

Hi,

handling of the MSRs in KVM is done entirely in the hypervisor.  QEMU
only gets/sets them in order to support migration.  You need to modify
the KVM kernel module for the VM to recognize your special MSRs.

Paolo