Re: [Qemu-devel] [PATCH v4 5/5] kvm/x86: Hyper-V kvm exit

['Roman Kagan']

On Fri, Dec 18, 2015 at 05:01:59PM +0100, Paolo Bonzini wrote:
> On 18/12/2015 16:19, Pavel Fedin wrote:
> > As far as i understand this code, KVM_EXIT_HYPERV is called when one
> > of three MSRs are accessed. But, shouldn't we have implemented 
> > instead something more generic, like KVM_EXIT_REG_IO, which would
> > work similar to KVM_EXIT_PIO or KVM_EXIT_MMIO, but carry register 
> > code and value?
> 
> Yes, we considered that.  There were actually patches for this as well.
>  However, in this case the register is still emulated in the kernel, and
> userspace just gets informed of the new value.
> 
> > This would allow us to solve the same task which we have done here,
> > but this solution would be reusable for other devices and other 
> > archirectures. What if in future we have more system registers to
> > emulate in userspace?
> 
> If we do get that, we will just rename KVM_EXIT_HYPERV to
> KVM_EXIT_MSR_ACCESS, and KVM_EXIT_HYPERV_SYNIC to
> KVM_EXIT_MSR_HYPERV_SYNIC, and struct kvm_hyperv_exit to kvm_msr_exit.

A generic implemenation will probably just convey (msr#, value) pair,
and KVM_EXIT_MSR_HYPERV_SYNIC wouldn't be needed at all.

I don't immediately see why it wouldn't work for us; we'd have reused
the infrastructure if it existed when we started our work.  I didn't see
Peter's patches yet; maybe we can come up with an interim solution to
fit in the merge window but expose a sufficiently generic API.

Roman.