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Re: [Xen-devel] Patches for stable

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.


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