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

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Date:	Mon, 25 Jan 2016 10:08:55 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Andrew Cooper <andrew.cooper3@...rix.com>,
	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	Juergen Gross <jgross@...e.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>, david.vrabel@...rix.com,
	hpa@...or.com, xen-devel@...ts.xenproject.org,
	Borislav Petkov <bp@...e.de>, roger.pau@...rix.com
Subject: Re: [Xen-devel] [PATCH v1 04/12] xen/hvmlite: Bootstrap HVMlite guest

On 01/22/2016 07:30 PM, Andrew Cooper wrote:
> On 22/01/2016 23:32, Luis R. Rodriguez wrote:
>> On Fri, Jan 22, 2016 at 04:35:50PM -0500, Boris Ostrovsky wrote:
>>> +	/*
>>> +	 * See Documentation/x86/boot.txt.
>>> +	 *
>>> +	 * Version 2.12 supports Xen entry point but we will use default x86/PC
>>> +	 * environment (i.e. hardware_subarch 0).
>>> +	 */
>>> +	xen_hvmlite_boot_params.hdr.version = 0x212;
>>> +	xen_hvmlite_boot_params.hdr.type_of_loader = 9; /* Xen loader */
>>> +}
>> I realize PV got away with setting up boot_params on C code but best
>> ask now that this new code is being introduced: why can't we just have
>> the Xen hypervisor fill this in? It'd save us all this code.
> I agree that this looks to be a mess.  Having said that, the DMLite boot
> protocol is OS agnostic, and will be staying that way.
>
> It happens to look suspiciously like multiboot; a flat 32bit protected
> mode entry (at a location chosen in an ELF note), with %ebx pointing to
> an in-ram structure containing things like a command line and module list.
>
> I would have though the correct way to do direct Linux support would be
> to have a very small init stub which constructs an appropriate zero
> page, and lets the native entry point get on with things.

Which is really what 
hvmlite_start_xen()->xen_prepare_hvmlite()->hvmlite_bootparams() is 
doing. Not much more than that (for 64-bit it also loads identity 
mapping because that's what startup_64 wants)

-boris

>
> This covers the usecase where people wish to boot a specific Linux
> kernel straight out of the dom0 filesystem.
>
> For the alternative usecase of general OS support, dom0 would boot
> something such as grub2 as the DMLite "kernel", at which point all
> stooging around in the guests filesystem is done from guest context,
> rather than control context (mitigating a substantial attack surface).
>
> ~Andrew