On Wed, Dec 02, 2020 at 01:06:28PM -0500, Peter Xu wrote:
> On Wed, Nov 25, 2020 at 12:44:07PM -0800, Elena Afanasova wrote:
>
> [...]
>
> > Wire protocol
> > -------------
> > The protocol spoken over the file descriptor is as follows. The device reads
> > commands from the file descriptor with the following layout::
> >
> > struct ioregionfd_cmd {
> > __u32 info;
> > __u32 padding;
> > __u64 user_data;
> > __u64 offset;
> > __u64 data;
> > };
>
> I'm thinking whether it would be nice to have a handshake on the wire protocol
> before starting the cmd/resp sequence.
>
> I was thinking about migration - we have had a hard time trying to be
> compatible between old/new qemus. Now we fixed those by applying the same
> migration capabilities on both sides always so we do the handshake "manually"
> from libvirt, but it really should be done with a real handshake on the
> channel, imho.. That's another story, for sure.
>
> My understanding is that the wire protocol is kind of a standalone (but tiny)
> protocol between kvm and the emulation process. So I'm thinking the handshake
> could also help when e.g. kvm can fallback to an old version of wire protocol
> if it knows the emulation binary is old. Ideally, I think this could even
> happen without VMM's awareness.
>
> [...]
I imagined that would happen in the control plane (KVM ioctls) instead
of the data plane (the fd). There is a flags field in
ioctl(KVM_SET_IOREGION):
struct kvm_ioregion {
__u64 guest_paddr; /* guest physical address */
__u64 memory_size; /* bytes */
__u64 user_data;
__s32 fd; /* previously created with KVM_CREATE_IOREGIONFD */
__u32 flags;
__u8 pad[32];
};
When userspace sets up the ioregionfd it can tell the kernel which
features to enable.
Feature availability can be checked through ioctl(KVM_CHECK_EXTENSION).
Do you think this existing mechanism is enough? It's not clear to me
what kind of additional negotiation would be necessary between the
device emulation process and KVM after the ioregionfd has been
registered?
> > Ordering
> > --------
> > Guest accesses are delivered in order, including posted writes.
>
> I'm wondering whether it should prepare for out-of-order commands assuming if
> there's no handshake so harder to extend, just in case there could be some slow
> commands so we still have chance to reply to a very trivial command during
> handling the slow one (then each command may require a command ID, too). But
> it won't be a problem at all if we can easily extend the wire protocol so the
> ordering constraint can be extended too when really needed, and we can always
> start with in-order-only requests.
Elena and I brainstormed out-of-order commands but didn't include them
in the proposal because it's not clear that they are needed. For
multi-queue devices the per-queue registers can be assigned different
ioregionfds that are handled by dedicated threads.
Out-of-order commands are only necessary if a device needs to
concurrently process register accesses to the *same* set of registers. I
think it's rare for hardware register interfaces to be designed like
that.
This could be a mistake, of course. If someone knows a device that needs
multiple in-flight register accesses, please let us know.
Stefan
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