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Re: [Xen-devel] HVMLite / PVHv2 - using x86 EFI boot entry

On Wed, Apr 13, 2016 at 12:25:03PM +0200, Roger Pau Monné wrote:
> On Wed, Apr 13, 2016 at 12:12:25AM +0200, Luis R. Rodriguez wrote:
> [...]
> > Also, x86 does have a history of short DT use. Just pointing that its there 
> > as
> > an option as well. I'll Cc you on some thread about that.
> I don't see how this is relevant to the conversation that's going on:

Its relevant as George brought up DT as a *reason* why ARM was able
to cope with no custom entry point...

> How many x86 hardware provide DT?

One. CE4100.


> I bet this is 0%.

That's slightly more than 0%.

> How many OSes can boot on x86 using DT? Linux maybe, certainly FreeBSD, 
> Windows or OpenBSD won't be able to boot at all when provided a DT on x86.

You guys seem to be taking these things too personal. 

Let me repeat, my goal is to ensure we review things without a bias. The points
you make here *now* are things I welcome to the discussion as reasons for
ruling out DT as ways to fine tune further semantics, its however by no means
something we should have discarded.

> Is Xen going to craft a DT for x86 based on ACPI?  No, because it can't parse
> the DSDT or other dynamic tables that contain the information about the
> devices in the system.

Again, DT was brought up by George as reason why ARM was able to cope
with no custom entry point. That's all. What you raise is a good point
to highlight but it does not mean we can't use it if we wanted to for
other things, for instance as an alternative to extending the x86 boot
protocol with custom things which we may need to enhance semantics
early in boot. If that is a stupid prospect lets highlight that and
rule it out.

> I would also like to point out that DT or not DT is not really the problem 
> here, the issue that George was trying to point out is that on x86 there's 
> some legacy hardware that's considered to be always there, so it's presence 
> is not signaled by ACPI, and HVMlite is _not_ emulating this hardware. It 
> doesn't matter if the hardware description comes from ACPI or DT, this 
> hardware is considered to be always present on PC compatible hardware.

x86 Xen PV guests are not alone.  I'm adding quirks we can use to address this
in a clean way now which turns out to be very useful for other custom x86


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