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

[Juergen Gross <jgross@xxxxxxxx>]

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.


Juergen

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