Re: [Xen-devel] Patches for stable

[Juergen Gross <jgross@xxxxxxxx>]

On 04/04/18 16:46, Greg KH wrote:
> On Wed, Apr 04, 2018 at 04:30:30PM +0200, Juergen Gross wrote:
>> On 04/04/18 16:27, Greg KH wrote:
>>> On Wed, Apr 04, 2018 at 12:38:43PM +0200, Juergen Gross wrote:
>>>> Please add the patches:
>>>>
>>>> commit 038bac2b02989acf1fc938cedcb7944c02672b9f upstream
>>>> commit dfc9327ab7c99bc13e12106448615efba833886b upstream
>>>> commit b17d9d1df3c33a4f1d2bf397e2257aecf9dc56d4 upstream
>>>>
>>>> to the 4.15 and 4.16 stable kernels.
>>>>
>>>> Those patches are needed to boot Linux as PVH guest on recent Xen.
>>>
>>> So a new feature?  Why is that ok for stable kernels?
>>
>> It works for kernels since at least 4.11 on Xen 4.10.
> 
> Great, so what commit caused this to fail?
> 
> So far, in reading those commits, it sounds like they are "make Linux
> work again due to changes in Xen".  That sounds like a pretty bad thing
> that Xen did, why do we have to fix up their mess?

Xen did nothing bad. It was the "old" kernel implementation which relied
on an assumption which happened to be true by accident. Xen had to be
changed in order to enable grub2 to support PVH mode.

The PVH interface specifies that the RSDP address is available via the
start_info structure handed over to the PVH boot entry. The Linux kernel
didn't look at that address, but used the legacy method scanning low
memory for the RSDP table. As soon as Xen moved the RSDP to a higher
address (which is covered by the PVH interface specification) the kernel
could no longer be booted.

So it was clearly a fault of the kernel not complying to the PVH
specification.

> 
>>>> In PVH mode there is no guarantee the kernel can find the RSDP table
>>>> at the legacy location in low memory, which is a requirement for the
>>>> kernel to boot successful without those patches.
>>>
>>> Why not just use newer kernels for new Xen features?  This really
>>> doesn't look like a bugfix to me, does it to you?
>>
>> It does. A working setup will no longer work if Xen version is upgraded
>> to 4.11.
> 
> Why isn't this a regression in Xen that they fix?  Why are we
> responsible for adding new kernel features to work on newer versions of
> Xen and backport them to older kernels?

In case a Linux user program relies on undocumented behavior of the
kernel (e.g. a register being non-zero on return from a syscall), does
the kernel have to support that behavior eternally? I don't think so.
This is a similar case.


Juergen

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