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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 11/20/2017 06:20 AM, Juergen Gross wrote:
> On 20/11/17 11:57, Andrew Cooper wrote:
>> 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.


As I mentioned in the other thread --- it will when we get to dom0 support.

>>>> 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?

FYI (and this is not directly related to this thread) there was a
discussion with KVM engineers and they may be interested in doing a
PVH-like boot, using Xen PVH entry point.

Adding Maran who is looking at this.

-boris

>>> 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.
> 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.
>
>> Doing this in grub seems like the more appropriate place IMO.
> I don't think so.
>
>
> Juergen
>
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