Re: [PATCH] KVM: x86: Reinstate userspace hypercall support

[Paolo Bonzini <pbonzini@redhat.com>]

On 30/11/20 23:42, David Woodhouse wrote:
> Yes, good point.
> 
> Xen*does*  allow one hypercall (HVMOP_guest_request_vm_event) from
> !CPL0 so I don't think I can get away with allowing only CPL0 like
> kvm_hv_hypercall() does.
> 
> So unless I'm going to do that filtering in kernel (ick) I think we do
> need to capture and pass up the CPL, as you say. Which means it doesn't
> fit in the existing kvm_run->hypercall structure unless I'm prepared to
> rename/abuse the ->hypercall.pad for it.
> 
> I'll just use Joao's version of the hypercall support, and add cpl to
> struct kvm_xen_exit.hcall. Userspace can inject the UD# where
> appropriate.

Or you can use sync regs perhaps.  For your specific use I don't expect 
many exits to userspace other than Xen hypercalls, so it should be okay 
to always run with KVM_SYNC_X86_REGS|KVM_SYNC_X86_SREGS in 
kvm_run->kvm_valid_regs.

Paolo