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

On 01/24/2018 07:06 AM, Juergen Gross wrote:
> 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@xxxxxxxxxx> 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@xxxxxxxx>
>>>>>>> Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>
>>>>>> 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?

AFAIK PVH is still considered a tech preview --- Linux or Xen.

>>>   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.

For backports we don't need bootparams changes, do we?

I suspect we can do as little as removing "#ifdef CONFIG_KEXEC" around
acpi_rsdp in drivers/acpi/osl.c and then assigning it the value in
pvh_start_info.rsdp_paddr. (I haven't tried it)


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