Re: [Xen-devel] Patches for stable

[Juergen Gross <jgross@xxxxxxxx>]

On 05/04/18 08:33, Greg KH wrote:
> On Wed, Apr 04, 2018 at 06:32:17PM +0200, Juergen Gross wrote:
>> On 04/04/18 17:42, Greg KH wrote:
>>> On Wed, Apr 04, 2018 at 05:12:32PM +0200, Juergen Gross wrote:
>>>> 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.
>>>
>>> But it worked previously, so you can't fault Linux here :)
>>>
>>> How many other operating systems broke with this change?
>>
>> None.
>>
>> BSD did it correctly. I guess Mini-OS doesn't count, as it is mostly
>> Xen-internal, but it was not hit by this change.
> 
> Xen doesn't support anything other than BSD, Linux, and Mini-OS? :)

No other OS supports PVH mode so far.

>>> Not at all.  We have a working kernel here.  Xen changed and broke
>>> working Linux systems.  Now I understand the goal of wanting to also
>>> change Linux to work properly, but these changes are really a new
>>> feature addition if you read the patches.
>>
>> We have a working kernel just by luck. Would your reasoning be the same
>> if the kernel would use an EFI runtime service wrong and an EFI update
>> would lead to a crash?
> 
> If a UEFI/BIOS update broken working systems, first we would go yell at
> the BIOS engineers for doing something foolish (like I am doing here...)
> Then we would grumble and go fix the issue in the latest kernel version
> and tell people to update to a new release and never buy from that
> vendor ever again as they obviously do not care about their users.

Even if the kernel wasn't using the EFI interfaces correctly and just
worked by accident? Sorry, that's ridiculous.

> So, I'll gladly tell everyone who hits this bug, to stop using Xen as
> they don't care about their users, and to work around it they have to
> use the 4.17 kernel release.

The kernel is wrong here. You don't want to take the patches fixing the
issue. That's rather sad as PVH mode was meant to replace PV in the
future, which will remove the need for most of the paravirt ops stuff.
You are just shifting that possibility some months further into the
future.

I won't fight against you any longer.


Juergen

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