Re: [Xen-devel] Patches for stable

[George Dunlap <dunlapg@xxxxxxxxx>]

On Fri, Apr 6, 2018 at 11:02 AM, Juergen Gross <jgross@xxxxxxxx> wrote:
> 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?

Why do you think it's 2.6 in the first place?  Because there are
user-space tools that depend on the kernel version being equal to
"2.6" which would break if that were changed.

*This* is the degree to which the Linux community tries to prevent
breaking existing systems -- because of a clear bug in userspace
tooling, they've kept the advertized kernel version the same for the
better part of a decade.

 -George

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