Anthony Liguori wrote:
Gregory Haskins wrote:
We add a generic hypercall() mechanism for use by IO code which is
compatible with a variety of hypervisors, but which prefers to use
hypercalls over other types of hypervisor traps for performance and/or
feature reasons.
For instance, consider an emulated PCI device in KVM. Today we can
chose
to do IO over MMIO or PIO infrastructure, but they each have their own
distinct disadvantages:
*) MMIO causes a page-fault, which must be decoded by the hypervisor
and is
therefore fairly expensive.
*) PIO is more direct than MMIO, but it poses other problems such as:
a) can have a small limited address space (x86 is 2^16)
b) is a narrow-band interface (one 8, 16, 32, 64 bit word at a
time)
c) not available on all archs (PCI mentions ppc as problematic)
and
is therefore recommended to avoid.
Hypercalls, on the other hand, offer a direct access path like PIOs, yet
do not suffer the same drawbacks such as a limited address space or a
narrow-band interface. Hypercalls are much more friendly to software
to software interaction since we can pack multiple registers in a way
that is natural and simple for software to utilize.
No way. Hypercalls are just about awful because they cannot be
implemented sanely with VT/SVM as Intel/AMD couldn't agree on a common
instruction for it. This means you either need a hypercall page,
which I'm pretty sure makes transparent migration impossible, or you
need to do hypercall patching which is going to throw off attestation.
This is irrelevant since KVM already does this patching today and we
therefore have to deal with this fact. This new work is riding on the
existing infrastructure so it of negligible cost to add new vectors at
least w.r.t. your concerns above.
If anything, I'd argue that we shouldn't use hypercalls for anything
in KVM because it will break run-time attestation.
The cats already out of the bag on that one. Besides, even if the
hypercall infrastructure does break attestation (which I don't think is
proven as fact) not everyone cares about migration. At some point,
someone may want to make a decision to trade performance for the ability
to migrate (or vice versa). I am ok with that.
Hypercalls cannot pass any data either. We pass data with hypercalls
by relying on external state (like register state).
This is a silly argument. The CALL or SYSENTER instructions do not pass
data either. Rather, they branch the IP and/or execution context,
predicated on the notion that the new IP/context can still access the
relevant machine state prior to the call. VMCALL type instructions are
just one more logical extension of that same construct.
PIO on the other hand is different. The architectural assumption is
that the target endpoint does not have access to the machine state.
Sure, we can cheat in virtualization by knowing that it really will
have access, but then we are still constrained by all the things I have
already mentioned that are disadvantageous to PIOs, plus
It's just as easy to do this with PIO.
..you are glossing over the fact that we already have the infrastructure
to do proper register setup in kvm_hypercallX(). We would need to come
up with an similar (arch specific) pio_callX() as well. Oh wait, no
we wouldn't, since PIOs apparently only work on x86. ;) Ok, so we would
need to come up with these pio_calls for x86, and no other arch can use
the infrastructure (but wait, PPC can use PCI too, so how does that
work? It must be either MMIO emulation or its not supported? That puts
us back to square one).
In addition, we can have at most 8k unique vectors on x86_64 (2^16/8
bytes per port).Even this is a gross overestimation because it
assumes the entire address space is available for pio_call, which it
isn't, and it assumes all allocations are in the precise width of the
address space (8-bytes), which they wont be. It doesn't exactly sound
like a winner to me.
VMware does this with vmport, for instance. However, in general, you
do not want to pass that much data with a notification. It's better
to rely on some external state (like a ring queue) and have the
hypercall act simply as a notification mechanism.
Disagree completely. Things like shared-memory rings provide a really
nice asynchronous mechanism, and for the majority of your IO and for
fast-path thats probably exactly what we should do. However, there are
plenty of patterns that fit better with a synchronous model. Case in
point: see VIRTIO_VBUS_FUNC_GET_FEATURES, SET_STATUS, and RESET
functions in the virtio-vbus driver I posted. And as an aside, the ring
notification rides on the same fundamental synchronous transport.
So sure, you could argue that you could also make an extra call ring,
place your request in the ring, and use a flush/block primitive to wait
for that item to execute. But that sounds a bit complicated when all I
want is the
