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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 20/11/17 10:43, Juergen Gross wrote:
> On 20/11/17 11:21, Andrew Cooper wrote:
>> On 20/11/17 10:04, Juergen Gross wrote:
>>> On 20/11/17 10:58, Andrew Cooper wrote:
>>>> On 20/11/2017 09:55, Juergen Gross wrote:
>>>>> On 20/11/17 10:51, Roger Pau Monné wrote:
>>>>>> Adding xen-devel, dropped it on my reply. 
>>>>>>
>>>>>>     Replying from my phone, sorry for the formatting. 
>>>>>>
>>>>>>
>>>>>>     El 20 nov. 2017 9:35, "Juergen Gross" <jgross@xxxxxxxx
>>>>>>     <mailto:jgross@xxxxxxxx>> escribió:
>>>>>>
>>>>>>         For PVH domains loading of the ACPI RSDP table is done via
>>>>>>         allocating
>>>>>>         a domain loader segment after having loaded the kernel. This
>>>>>>         leads to
>>>>>>         the RSDP table being loaded at an arbitrary guest address 
>>>>>> instead of
>>>>>>         the architectural correct address just below 1MB. 
>>>>>>
>>>>>>
>>>>>>     AFAIK this is only true for legacy BIOS boot, when using UEFI the
>>>>>>     RSDP can be anywhere in memory, hence grub2 must already have an
>>>>>>     alternative way of finding the RSDP apart from scanning the low 1MB.
>>>>> The problem isn't grub2, but the loaded linux kernel. Without this
>>>>> patch Linux won't find the RSDP when booted in a PVH domain via grub2.
>>>>>
>>>>> I could modify grub2 even further to move the RSDP to the correct
>>>>> address, but I think doing it correctly on Xen side is the better
>>>>> option.
>>>> Why?  The PVH info block contains a pointer directly to the RSDP, and
>>>> Linux should be following this rather than scanning for it using the
>>>> legacy method.
>>> Oh no, please not this discussion again.
>>>
>>> We already had a very long discussion how to do PVH support in grub2,
>>> and the outcome was to try to use the standard boot entry of the kernel
>>> instead the PVH sepcific one.
>>>
>>> The Linux kernel right now doesn't make use of the RSDP pointer in the
>>> PVH info block, so I think we shouldn't change this when using grub2.
>> I clearly missed the previous discussion, and I don't advocate using yet
>> another PVH-specific entry point, but how does Linux cope in other
>> non-BIOS environments?  Does it genuinely rely exclusively on the legacy
>> mechanism?
> Looking at the code I think so, yes. Maybe there are cases where no RSDP
> is needed, but in the grub2/PVH case we need it to distinguish PVH from
> HVM.

In which case, being a Linux limitation, I think it is wrong to
unilaterally apply this restriction to all other PVH guests.

Doing this in grub seems like the more appropriate place IMO.

~Andrew

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