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

[Juergen Gross <jgross@xxxxxxxx>]

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.

Juergen

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