Re: [PATCH v2 1/7] KVM:VMX: Define CET VMCS fields and bits

[Sean Christopherson]

On Tue, Jan 29, 2019 at 04:19:34PM +0100, Paolo Bonzini wrote:
> On 28/01/19 11:33, Yang Weijiang wrote:
> >> There is no code in this series to pass these fields to and from
> >> userspace, and also to save/restore U_CET, INT_SSP_TAB, PL0_SSP and
> >> PL3_SSP across context switches.
> >>
> > The kernel consumes these MSRs, please see kernel CET patch:
> > https://lkml.org/lkml/fancy/2018/11/20/225
> 
> Still, even if the kernel saves these fields across context switch in
> XSAVE areas, KVM must support accesses to the MSRs from userspace, for
> example in order to perform live migration.
> 
> For example, when reading/writing these in kvm_set_msr or
> kvm_get_msr_common, you would have to do a read/write from the host
> MSRs.  You also have to put kvm_load_guest_fpu/kvm_put_guest_fpu calls
> in __msr_io.

Paolo, can you elaborate on why KVM would read the host MSRs?  Wouldn't
kvm_{get,set}_msr() pull the values from the VMCS when necessary?