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Re: [Xen-devel] RFC Userspace hypercalls

On Wed, 2016-01-06 at 11:44 +0000, Andrew Cooper wrote:
> All console logging is synchronous (to ensure that log messages have
> escaped the VM before an action occurs) and by default, an HVM test will
> use the qemu debug port, console_io hypercall, and PV console (which
> uses evtchn hypercalls).

All three simultaneously, or it picks one depending on the scenario?

> There are already scenarios under test where we cannot rely on the test
> kernel having a fully functioning set of entry points (e.g. the DPL part
> of the test above).ÂÂTherefore I specifically want to make it possible
> to make userspace hypercalls, rather than simply making them possible to
> be trapped-and-forwarded.

And in these test cases there is useful logging to be done between the
break the world and repair the world phases which I suppose follows if
things didn't crash?

> As a result, I proposing introducing a hypercall which allows a domain
> to adjust its entry criteria for hypercalls (e.g. set_hypercall_iopl). 
> Doing this for HVM guests is straight forward, but PV guests are harder,
> as they bounce through Xen entrypoints.
> For PV guests, I propose that userspace hypercalls get implemented with
> the int $0x82 path exclusively.ÂÂi.e. enabling userspace hypercalls
> causes the hypercall page writing logic to consider the guest a ring1
> kernel, and the int $0x82 entrypoint suitably delegates between a
> regular hypercall and a compat hypercall.
> Thoughts?

Would a xenconsoled mode which polls for updates (on specific guests only),
along with the guest spinning waiting for the cons pointer to catch the
prod one if it cares about synchronous logging be sufficient for this use

Other random ideas:
Implement the debug io port for PV guests too
Log to a in guest buffer, as David suggested, possibly use xenaccess or
similar to trap updates or as a doorbell.

> ~Andrew
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