Re: [PATCH RFC 11/39] KVM: x86/xen: evtchn signaling via eventfd

[David Woodhouse <dwmw2@infradead.org>]

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On Mon, 2020-11-30 at 15:08 +0000, Joao Martins wrote:
> On 11/30/20 12:55 PM, David Woodhouse wrote:
> > On Mon, 2020-11-30 at 12:17 +0000, Joao Martins wrote:
> > > On 11/30/20 9:41 AM, David Woodhouse wrote:
> > > > On Wed, 2019-02-20 at 20:15 +0000, Joao Martins wrote:
> > > One thing I didn't quite do at the time, is the whitelisting of unregistered
> > > ports to userspace. Right now, it's a blacklist i.e. if it's not handled in
> > > the kernel (IPIs, timer vIRQ, etc) it goes back to userspace. When the only
> > > ones which go to userspace should be explicitly requested as such
> > > and otherwise return -ENOENT in the hypercall.
> > 
> > Hm, why would -ENOENT be a fast path which needs to be handled in the
> > kernel?
> > 
> 
> It's not that it's a fast path.
> 
> Like sending an event channel to an unbound vector, now becomes an possible vector to
> worry about in userspace VMM e.g. should that port lookup logic be fragile.
> 
> So it's more along the lines of Nack-ing the invalid port earlier to rather go
> to go userspace to invalidate it, provided we do the lookup anyway in the kernel.

If the port lookup logic is fragile, I *want* it in the sandboxed
userspace VMM and not in the kernel :)

And unless we're going to do *all* of the EVTCHNOP_bind*, EVTCHN_close,
etc. handling in the kernel, doesn't userspace have to have all that
logic for managing the port space anyway?

I think it's better to let userspace own it outright, and use the
kernel bypass purely for the fast paths. The VMM can even implement
IPI/VIRQ support in userspace, then use the kernel bypass if/when it's
available.

> > > Perhaps eventfd could be a way to express this? Like if you register
> > > without an eventfd it's offloaded, otherwise it's assigned to userspace,
> > > or if neither it's then returned an error without bothering the VMM.
> > 
> > I much prefer the simple model where the *only* event channels that the
> > kernel knows about are the ones it's expected to handle.
> > 
> > For any others, the bypass doesn't kick in, and userspace gets the
> > KVM_EXIT_HYPERCALL exit.
> > 
> 
> /me nods
> 
> I should comment on your other patch but: if we're going to make it generic for
> the userspace hypercall handling, might as well move hyper-v there too. In this series,
> I added KVM_EXIT_XEN, much like it exists KVM_EXIT_HYPERV -- but with a generic version
> I wonder if a capability could gate KVM_EXIT_HYPERCALL to handle both guest types, while
> disabling KVM_EXIT_HYPERV. But this is probably subject of its own separate patch :)

There's a limit to how much consolidation we can do because the ABI is
different; the args are in different registers.

I do suspect Hyper-V should have marshalled its arguments into the
existing kvm_run->arch.hypercall and used KVM_EXIT_HYPERCALL but I
don't think it makes sense to change it now since it's a user-facing
ABI. I don't want to follow its lead by inventing *another* gratuitous
exit type for Xen though.



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