On Tue, Aug 18, 2009 at 01:45:05PM +0300, Avi Kivity wrote:
> On 08/18/2009 01:28 PM, Michael S. Tsirkin wrote:
>>
>>> Suppose a nested guest has two devices. One a virtual device backed by
>>> its host (our guest), and one a virtual device backed by us (the real
>>> host), and assigned by the guest to the nested guest. If both devices
>>> use hypercalls, there is no way to distinguish between them.
>>>
>> Not sure I understand. What I had in mind is that devices would have to
>> either use different hypercalls and map hypercall to address during
>> setup, or pass address with each hypercall. We get the hypercall,
>> translate the address as if it was pio access, and know the destination?
>>
>
> There are no different hypercalls. There's just one hypercall
> instruction, and there's no standard on how it's used. If a nested call
> issues a hypercall instruction, you have no idea if it's calling a
> Hyper-V hypercall or a vbus/virtio kick.
userspace will know which it is, because hypercall capability
in the device has been activated, and can tell kernel, using
something similar to iosignalfd. No?
> You could have a protocol where you register the hypercall instruction's
> address with its recipient, but it quickly becomes a tangled mess.
I really thought we could pass the io address in register as an input
parameter. Is there a way to do this in a secure manner?
Hmm. Doesn't kvm use hypercalls now? How does this work with nesting?
For example, in this code in arch/x86/kvm/x86.c:
switch (nr) {
case KVM_HC_VAPIC_POLL_IRQ:
ret = 0;
break;
case KVM_HC_MMU_OP:
r = kvm_pv_mmu_op(vcpu, a0, hc_gpa(vcpu, a1, a2), &ret);
break;
default:
ret = -KVM_ENOSYS;
break;
}
how do we know that it's the guest and not the nested guest performing
the hypercall?
> And for what? pio and hypercalls have the same performance characteristics.
No idea about that. I'm assuming Gregory knows why he wants to use hypercalls,
I was just trying to help find a way that is also palatable, and flexible.
> --
> error compiling committee.c: too many arguments to function
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