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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.11.17 at 17:59, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 11/20/2017 11:43 AM, Jan Beulich wrote:
>>>>> On 20.11.17 at 17:28, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> On 11/20/2017 11:26 AM, Jan Beulich wrote:
>>>>>>> On 20.11.17 at 17:14, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>>> 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)?
>>>> Well, the EBDA (see the B in its name) is again something that's
>>>> meaningless without there being a BIOS.
>>> Exactly. So it should always be available for grub to copy the pointer
>>> there.
>> But what use would it be if grub copied it there? It just shouldn't
>> be there, neither before nor after grub (just like grub doesn't
>> introduce firmware into the system).
> 
> So that the guest can find it using standard methods. If Xen can't
> guarantee ACPI-compliant placement of the pointer then someone has to
> help the guest find it in the expected place. We can do it with a
> dedicated entry point by setting the pointer explicitly (although
> admittedly this is not done correctly now) or we need to have firmware
> (grub2) place it in the "right" location.
> 
> (It does look a bit hacky though)

Indeed. Of course ACPI without any actual firmware is sort of odd,
too. As to dedicated entry point and its alternatives: Xen itself
tells grub (aiui we're talking about a flavor of it running PVH itself)
where the RSDP is. Why can't grub forward that information in a
suitable way (e.g. via a new tag, or - for Linux - as a new entry
in the Linux boot header)?

Jan


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