Re: [PATCH v2] libxl: put RSDP for PVH guest near 4GB

From: Juergen Gross <jgross@suse.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>,
	"George Dunlap" <dunlapg@umich.edu>
Cc: xen-devel <xen-devel@lists.xenproject.org>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Wei Liu <wei.liu2@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>
Subject: Re: [PATCH v2] libxl: put RSDP for PVH guest near 4GB
Date: Wed, 24 Jan 2018 13:06:13 +0100	[thread overview]
Message-ID: <ee015869-6b7d-c12a-4f2f-266c8bccb2d8@suse.com> (raw)
In-Reply-To: <20180124105453.xlaech23qkl6ebni@MacBook-Pro-de-Roger.local>

On 24/01/18 11:54, Roger Pau Monné wrote:
> On Wed, Jan 24, 2018 at 10:42:39AM +0000, George Dunlap wrote:
>> On Wed, Jan 24, 2018 at 2:41 AM, Boris Ostrovsky
>> <boris.ostrovsky@oracle.com> wrote:
>>> On 01/18/2018 05:33 AM, Wei Liu wrote:
>>>> On Thu, Jan 18, 2018 at 11:31:32AM +0100, Juergen Gross wrote:
>>>>> Wei,
>>>>>
>>>>> On 01/12/17 15:14, Juergen Gross wrote:
>>>>>> Instead of locating the RSDP table below 1MB put it just below 4GB
>>>>>> like the rest of the ACPI tables in case of PVH guests. This will
>>>>>> avoid punching more holes than necessary into the memory map.
>>>>>>
>>>>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>>>>> Acked-by: Wei Liu <wei.liu2@citrix.com>
>>>>> Mind applying this one?
>>>> Don't worry, it is in my queue.
>>>>
>>>> Will come to this and other patches I accumulated soon.
>>>>
>>>> Wei.
>>>
>>> This requires kernel changes, doesn't it?
>>>
>>> https://lists.xenproject.org/archives/html/xen-devel/2017-12/msg00714.html
>>>
>>> And this series apparently never made it to the tree.
>>>
>>> PVH guests are broken now on staging.
>>
>> And the Linux side of PVH is officially supported now, right?  Which
>> means we need to have some way of being backwards compatible with
>> kernels without that patch, even if it does get  backported.
> 
> Linux had a bug that on PVH it expected to find the RSDP at the low
> 1MiB. The PVHv2 boot protocol has _never_ mentioned that the RSDP
> should be below 1MiB. There's just one comment in the start_info
> header that says:
> 
> "Xen on x86 will always try to place all the data below the 4GiB
> boundary."
> 
> And the RSDP address must be fetched from the rsdp_paddr field of the
> start_info.
> 
> I agree that the fact that Linux is broken is bad, but should not
> force us to change the protocol. What's more, when booting a PVHv2
> Dom0 the RSDP _must_ not be searched in the low 1MiB, or else Dom0 is
> going to find the firmware provided RSDP and everything is going to
> blow up.

Right.

My patches to repair this in the Linux kernel are on hold as they are
touching the boot parameters between grub2 and the kernel, too. And
this caused the kernel no longer booting in some cases as most distros
are shipping a grub2 breaking this interface via a non-upstream patch.

> IMO, Linux must be fixed and the patch backported up to 4.11.

I'll try soon.

Juergen

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