Re: [Xen-devel] Patches for stable

[Juergen Gross <jgross@xxxxxxxx>]

On 05/04/18 16:56, George Dunlap wrote:
> 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.

... whose memory is >=1GiB ...

> Option 2: Put the RSDP in highmem regardless
> Pro: Existing BSD instances whose memory is an exact multiple of 1GiB

... whose memory is >=1GiB ...

> will have slightly faster TLB miss times
> Con: Existing Linux instances don't boot at all

Option 3: add a config item to domain config for selecting the RSDP
          placement, defaulting to highmem (my patch)
Pro: Existing BSD and new Linux instances whose memory is >=1GiB will
     have slightly faster TLB miss times
Pro: Existing Linux instances can be made bootable by adding a new
     item to their domain config

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

Greg and stable removed.


Juergen

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