Re: [Xen-devel] [PATCH][v2] xen: Pass the location of the ACPI RSDP to DOM0.

[Philip Wernersbach <philip.wernersbach@xxxxxxxxx>]

On Tue, Jan 21, 2014 at 4:51 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>> On 20.01.14 at 23:08, Philip Wernersbach <philip.wernersbach@xxxxxxxxx> 
>>>> wrote:
>> xen: [v2] Pass the location of the ACPI RSDP to DOM0.
>>
>> Some machines, such as recent IBM servers, only allow the OS to get the
>> ACPI RSDP from EFI. Since Xen nukes DOM0's ability to access EFI, DOM0
>
> This reads as if this was a bug in Xen, which it isn't. Dom0's
> lack of EFI support when running on top of Xen is the issue.

It all depends on how you look at it, as there's two ways to fix this.
Linux currently supports EFI just fine. Xen nukes DOM0's ability to
access EFI using the current methods, which causes Linux's existing
EFI support to fail. This would be Xen's fault. If Xen currently
exposes EFI to DOM0 in some other way that Linux is not aware of, then
this would be Linux's fault. Either way, we can either choose to fix
Xen or fix Linux.

> I think I had indicated my opposition to this sort of hack on v1
> already; I'm not sure I asked which OSes usable as Dom0 but
> other than Linux recognize this option. Or which versions of
> Linux actually do (I'm pretty sure older ones don't).
>
> Bottom line - I continue to think that the issue should be fixed
> in Linux.

The method that this patch uses is a valid way to fix this. In the
best case, the DOM0 OS recognizes this option and uses it. In the
worst case, the DOM0 OS won't recognize the option and will ignore it,
so we're no worse off.

I agree with you that this is more of a stop gap than a long term fix.
The final solution is to fully expose EFI services to the DOM0 in some
way. However, getting there will take some time. The reason this patch
came about is that the company I work for bought new IBM servers in
the hope of migrating our existing Xen server farm to the new IBM
servers. But we soon found out that DOM0 couldn't find the ACPI RSDP
pointer on the new IBM servers, which means that Xen is dead in the
water on these machines for now. The final solution of exposing EFI to
the DOM0 is the ultimate goal, but for now businesses need an
immediate solution. This provides an acceptable solution until DOM0
EFI services are implemented at a later date. There is no reason why
this can't be merged now and then removed when DOM0 EFI service
support arrives.

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