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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.11.17 at 16:24, <jgross@xxxxxxxx> wrote:
> On 20/11/17 15:20, Jan Beulich wrote:
>>>>> On 20.11.17 at 15:14, <jgross@xxxxxxxx> 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. 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.
>>>
>>> Would this wording be okay?
>> 
>> To be honest (and in case it wasn't sufficiently clear form my
>> earlier replies) - I'm pretty much opposed to this below-1Mb thing.
>> There ought to be just plain RAM there for PVH.
> 
> So without my patch the RSDP table is loaded e.g. at about 6.5MB when
> I'm using grub2 (the loaded grub image is about 5.5MB in size and it
> is being loaded at 1MB).
> 
> When I'm using the PVH Linux kernel directly RSDP is just below 1MB
> due to pure luck (the bzImage loader is still using the PV specific
> ELF notes and this results in the loader believing RSDP is loadable
> at this address, which is true, but the tests used to come to this
> conclusion are just not applicable for PVH).
> 
> So in your opinion we should revoke the PVH support from Xen 4.10,
> Linux and maybe BSD because RSDP is loaded in middle of RAM of the
> guest?

So what's wrong with it being put wherever the next free memory
location is being determined to be by the loader, just like is being
done for other information, including modules (if any)?

> Doing it in a proper way you are outlining above would render
> current PVH guests unusable.

I'm afraid I don't understand which outline of mine you refer to.
Iirc all I said was that placing it below 1Mb is bogus. Top of RAM
(as I think I saw being mentioned elsewhere) probably isn't much
better. Special locations should be used only if there's really no
other way to convey information.

Jan


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