Re: [Xen-devel] Patches for stable

[George Dunlap <dunlapg@xxxxxxxxx>]

On Thu, Apr 5, 2018 at 3:09 PM, Juergen Gross <jgross@xxxxxxxx> wrote:
> On 05/04/18 15:42, George Dunlap wrote:
>> On Thu, Apr 5, 2018 at 2:06 PM, Juergen Gross <jgross@xxxxxxxx> wrote:
>>> On 05/04/18 15:00, Boris Ostrovsky wrote:
>>>> On 04/05/2018 08:19 AM, Juergen Gross wrote:
>>>>> On 05/04/18 12:06, George Dunlap wrote:
>>>>>
>>>>>> Aren't there flags in the binary somewhere that could tell the
>>>>>> toolstack / Xen whether the kernel in question needs the RSDP table in
>>>>>> lowmem, or whether it can be put higher?
>>>>> Not really. Analyzing the binary whether it accesses the rsdp_addr in
>>>>> the start_info isn't the way to go, IMO.
>>>>>
>>>>> I've sent a patch to xen-devel adding a quirk flag to the domain's
>>>>> config to enable the admin special casing such an "old" kernel.
>>>>
>>>> Can we backport latest struct hvm_start_info changes (which bumped
>>>> interface version) to 4.11 and pass RSDP only for versions >=1?
>>>
>>> And this would help how?
>>>
>>> RSDP address is passed today, the kernel just doesn't read it. And
>>> how should Xen know which interface version the kernel is supporting?
>>> And Xen needs to know that in advance in order to place the RSDP in
>>> low memory in case the kernel isn't reading the RSDP address from
>>> start_info.
>>
>> But the kernel image has ELF notes, right?  You can put one that
>> indicates that this binary *does* know how to read the RSDP from the
>> start_info, and if you don't find that, put it in lowmem.
>
> Sow you would hurt BSD which does read the RSDP address correctly but
> (today) has no such ELF note.
>
> I think extending the PVH interface in such a way is no good idea.

Option 1: Put the RSDP in lowmem unless we know the guest will use the
address in start_info
Pro: Existing Linux instances boot
Con: Existing BSD instances whose memory is an exact multiple of 1 GiB
will have slightly slower TLB miss times.

Option 2: Put the RSDP in highmem regardless
Pro: Existing BSD instances whose memory is an exact multiple of 1GiB
will have slightly faster TLB miss times
Con: Existing Linux instances don't boot at all

This seems like a no-brainer to me.  But anyway, maybe we should move
the discussion elsewhere and stop bothering Greg. :-)

 -George

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