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Re: [Xen-devel] Patches for stable

On 06/04/18 11:49, George Dunlap wrote:
> On Thu, Apr 5, 2018 at 7:33 PM, Boris Ostrovsky
> <boris.ostrovsky@xxxxxxxxxx> wrote:
>> On 04/05/2018 01:11 PM, Juergen Gross wrote:
>>> 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.
>> This can be predicated on
>>     ELFNOTE(Xen, XEN_ELFNOTE_GUEST_OS,       .asciz "linux")
>> BSD will behave as it does now. For linux we could add feature flag (or
>> errata flag). Unfortunately I don't see a way to extract major.minor
>> from the headers, otherwise we could use that.
> OTOH, one advantage of having a separate elfnote, rather than gating
> it on Linux version, is that if a distro wanted to, they could do
> their own backport to (say) Linux 4.15 and reap the advantages.

Hmm, Linux kernel has already an elfnote with the guest version. It is
set to "2.6". What about writing the actual kernel version into that
note and assume everything != "2.6" to support a high RSDP address?


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