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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 20/11/17 10:04, 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?

~Andrew

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