Re: [Xen-devel] [PATCH for-4.10] libxc: load acpi RSDP table at correct address

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 11/20/2017 10:27 AM, Juergen Gross wrote:
> On 20/11/17 15:25, Boris Ostrovsky wrote:
>> On 11/20/2017 09:14 AM, Juergen Gross wrote:
>>> On 20/11/17 14:56, Boris Ostrovsky wrote:
>>>> On 11/20/2017 06:50 AM, Jan Beulich wrote:
>>>>>>>> On 20.11.17 at 12:20, <jgross@xxxxxxxx> wrote:
>>>>>> Which restriction? I'm loading the RSDP table to its architectural
>>>>>> correct addres if possible, otherwise it will be loaded to the same
>>>>>> address as without my patch. So I'm not adding a restriction, but
>>>>>> removing one.
>>>>> What is "architecturally correct" in PVH can't be read out of
>>>>> specs other than what we write down. When there's no BIOS,
>>>>> placing anything right below the 1Mb boundary is at least
>>>>> bogus.
>>>> Unless it's a UEFI boot -- where else would you put it? Aren't these two
>>>> (UEFI and non-UEFI) the only two options that the ACPI spec provides?
>>> I think Jan is right: for PVH its _our_ job to define the correct
>>> placement. 
>> Yes, and if it is placed in a non-standard location then the guest will
>> have to deal with it in a non-standard way. Which we can in Linux by
>> setting acpi_rsdp pointer in the special PVH entry point, before jumping
>> to Linux "standard" entry --- startup_{32|64}().
>>
>> But if your goal is to avoid that special entry point (and thus not set
>> acpi_rsdp) then how do you expect kernel to find RSDP?
>>
>>> Which still can be the same as in the BIOS case, making
>>> it easier to adapt any guest systems.
>>>
>>> So I'd say: The RSDP address in PVH case is passed in the PVH start
>>> info block to the guest. In case there is no conflict with the
>>> physical load address of the guest kernel the preferred address of
>>> the RSDP is right below the 1MB boundary.
>> And what do we do if there *is* a conflict?
> Either as without my patch: use the first available free memory page.
>
> Or: add a domain config parameter for specifying the RSDP address
> (e.g. default: as today, top: end of RAM, legacy: just below 1MB, or
> a specific value) and fail to load in case of a conflict.

This feels like a band-aid to work around a problem that we want to fix
in the long term anyway.

What could cause grub2 to fail to find space for the pointer in the
first page? Will we ever have anything in EBDA (which is one of the
possible RSDP locations)?

-boris


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