Bjorn Helgaas wrote:
On Tuesday 24 March 2009 01:05:43 am Jeremy Fitzhardinge wrote:
Tian, Kevin wrote:
OK, then it's safe to avoid that change. I had thought that dom0's 0-1M
is identity-mapped to machine 0-1M... :-)
No, only the ISA 640k-1M region.
I'm speaking out of turn here because I don't work on Xen or
suspend/resume. However, I do try to clean up random bits of
ACPI, and I have to say this patch looks like a pain in the
maintenance department. Having tests for a specific hypervisor
is unpleasant. We don't want to end up with tests for a collection
of hypervisors.
I agree. If we start to see other hypervisor-specific changes in this
area, we'd need to rethink this approach. But I'm not inclined to add a
layer of infrastructure to just deal with Xen. (IOW, abstract only when
there's something to abstract.)
It looks like suspend becomes a weird hybrid of
ACPI and Xen, which makes it harder to reason about future suspend
changes. And all this discussion about 640k-1M and dom0 identity
mapping and "there's no special effort to remap it" and whether
there are conflicts makes me nervous. There's no way all those
assumptions can be remembered or verified five years down the road.
Well, I think Kevin was over-complicating things in his own mind. The
upshot is that the normal "running on bare metal" code can do its normal
thing, and if we happen to be running under Xen we can make it a no-op.
In other words, the acpi developers don't really need to worry about the
"running under Xen case", for the most part.
The two changes this patch makes are, unfortunately, unavoidable:
1. Turn the final "enter sleep" into a hypercall, so that Xen can do
all the low-level context save/restore. The normal baremetal case
is still localized in acpica/hwsleep.c, but it (may) make an
excursion into arch code to see if it should do something else, and
2. Directly enter the sleep state, rather than save cpu context
(which Xen does)
Though, come to think of it, perhaps there's no harm in letting the
kernel do its own state-saving. I'll check.
J
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