How to get a few MSR values from userspace?

[Razvan Cojocaru <rzvncj@gmail.com>]

Hello,

I need access to some MSR values that are not currently being saved in 
struct hvm_hw_cpu. Among them are MSR_IA32_MC0_CTL, MSR_IA32_MISC_ENABLE 
and MSR_IA32_ENERGY_PERF_BIAS.

The way I'm approaching this that I'll patch xen/arch/x86/hvm/vmx/vmx.c 
and xen/arch/x86/hvm/svm/svm.c, and add this in vmx_save_cpu_state() and 
svm_save_cpu_state(), respectively:

hvm_msr_read_intercept(MSR_IA32_MC0_CTL, &data->msr_mc0_ctl);

and so on, for the other registers (after adding the msr_mc0_ctl member 
to struct hvm_hw_cpu, of course). I would also have to do the reverse 
operation (using hvm_msr_write_intercept()) in vmx_load_cpu_state().

My questions:

1. Does it seem architecturally sound to perform the described 
modifications? Can I use hvm_msr_xxx_intercept() for both the VMX and 
the SVM code?

2. It seems repetitive to have duplicated code in both 
vmx_save_cpu_state() and svm_save_cpu_state(), does it make more sense 
to have it like that anyway (in case, for example, the SVM way to 
retrieve that register could change in the future)?

3. Do I need to do additional things so that I won't break anything else?

4. Is there a better way to achieve what I'm after?

Thanks,
Razvan Cojocaru