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

[Paolo Bonzini]

On 04/03/19 19:56, Sean Christopherson wrote:
> 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?

Not all MSRs are in the VMCS; IA32_U_CET and IA32_PL*_SSP are not.

Paolo