Re: [Xen-devel] Patches for stable

[George Dunlap <dunlapg@xxxxxxxxx>]

On Fri, Apr 6, 2018 at 11:57 AM, Juergen Gross <jgross@xxxxxxxx> wrote:
> On 06/04/18 12:07, George Dunlap wrote:
>> 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.
>
> Can you give me a hint where this would be? The elfnote is being fed
> into elf_dom_parms->guest_ver. I couldn't find any reference to that
> other than setting it.
>
> The other reference I could find is the readnotes utility. In the Xen
> tree I couldn't find any tool using the output of that.
>
>> *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.
>
> You are aware of the fact I'm speaking of a Xen-specific elfnote?

No I wasn't.

FWIW I think taking "I have set the kernel version correctly" to mean
"I know to read the address of the RSDP table from the start_info
page" isn't a very good idea.  For one, it's fragile: someone may not
realize that the one implies the other.  Secondly, it may turn out
that there's a reason it's been kept at "2.6", and then we'd have to
revert the one change and make a new elfnote anyway.

But I don't know enough about this particular area to argue strongly
one way or the other.

 -George

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