On 08/18/2009 02:07 PM, Michael S. Tsirkin wrote:
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?
The host kernel sees a hypercall vmexit. How does it know if it's a
nested-guest-to-guest hypercall or a nested-guest-to-host hypercall?
The two are equally valid at the same time.
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?
The host knows whether the guest or nested guest are running. If the
guest is running, it's a guest-to-host hypercall. If the nested guest
is running, it's a nested-guest-to-guest hypercall. We don't have
nested-guest-to-host hypercalls (and couldn't unless we get agreement on
a protocol from all hypervisor vendors).
--
error compiling committee.c: too many arguments to function
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