linux-kernel - Re: [PATCH v1 04/12] xen/hvmlite: Bootstrap HVMlite guest

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Date:	Tue, 26 Jan 2016 14:00:09 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Andy Lutomirski <luto@...capital.net>,
	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Roger Pau Monné <roger.pau@...rix.com>,
	Juergen Gross <jgross@...e.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Borislav Petkov <bp@...e.de>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [PATCH v1 04/12] xen/hvmlite: Bootstrap HVMlite guest

On 01/26/2016 01:46 PM, Andy Lutomirski wrote:
> On Tue, Jan 26, 2016 at 10:34 AM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
>> On Mon, Jan 25, 2016 at 05:28:08PM -0500, Boris Ostrovsky wrote:
>>> On 01/25/2016 04:21 PM, H. Peter Anvin wrote:
>>>> On 01/25/16 13:12, Luis R. Rodriguez wrote:
>>>>>> Perhaps, but someone would still have to set hardware_subarch. And
>>>>>> it's hvmlite_bootparams() that does it.
>>>>> No, Xen would do it as well, essentially all of hvmlite_bootparams() could be
>>>>> done in Xen.
>>>>>
>>>> Or a stub code.
>>> This patch in fact is the stub for Xen HVMlite guests, after we are
>>> done with it we jump to bare-metal startup code (i.e startup_32|64)
>> Right the point is the stub need not be in Linux, I'll explain in the other
>> thread where I provided more details on the different known approaches.
>>
> ISTM if the Xen ABI-specified entry point has a different convention
> than the Linux native entry, then the stub should live in Linux.  It
> would be just a couple if lines of code, right?

It's not exactly a couple of lines but it's not large neither. It mainly 
sets up boot_params (similar to what make_boot_params() does for EFI). 
Plus, for 64-bit, it loads identity page tables and switches to long 
mode. And then jumps to bare-meta startup code.


>
> The issue that caused headaches in the past isn't that there's code
> that's executed only on native, it's that there are whole big
> functions that are executed only on native for no good reason and that
> aren't clearly marked.

This won't happen with HVMlite.

>
> If we had native_start_kernel and xen_start_kernel, and they both
> called very quickly in to common_start_kernel, it would be very clear
> what's going on.

What is now xen_start_kernel() is no longer the entry point for HVMlite. 
It is called as x86_init.oem.arch_setup() (or I may even move it to 
x86_hyper_xen.init_platform() or something like that).

-boris