Re: [PATCH] pc-bios: s390x: Add a comment to the io and external new PSW setup

[Thomas Huth]

On 22/07/2020 10.05, Janosch Frank wrote:
> On 7/22/20 9:39 AM, Christian Borntraeger wrote:
>>
>>
>> On 22.07.20 09:24, Janosch Frank wrote:
>>> On 7/22/20 8:43 AM, Christian Borntraeger wrote:
>>>>
>>>>
>>>> On 15.07.20 16:08, Janosch Frank wrote:
>>>>> Normally they don't need to be set up before waiting for an interrupt
>>>>> but are set up on boot. The BIOS however might overwrite the lowcore
>>>>> (and hence the PSWs) when loading a blob into memory and therefore
>>>>> needs to set up those PSWs more often.
>>>>
>>>> Now when I read the new comment this actually inidicates a bug. 
>>>> When do we restore the original content? If the loaded program
>>>> does have interrupt handlers in the original image and relies on that
>>>> then we are broken, no?
>>>
>>> I haven't seen references to a save/restore functionality for those
>>> PSWs. And I also think it's not that easy to do because we have multiple
>>> ways of loading data and if we want to print when loading we might end
>>> up overwriting and then saving the written value for a later restore.
>>>
>>> I need to have a closer look at how virtio works, but wouldn't we have a
>>> chicken - egg problem with IO interrupts for IO that writes the prefix?
>>>
>>> The BIOS often has "interesting" solutions to problems.
>>> If you have a quick fix, be my guest and send it. If not I'd put it on
>>> my todo list or let Stefan make it a proper dev item.
>>
>> Maybe a global fixup table in BIOS memory that restores all the memory that
>> we messed with when we hand over control? Can you at least change the comment
>> here to add a fixme?
> 
> Sure

Hi Janosch!

Did you ever send a new version of this patch with the FIXME added? I
can't find it right now... Or shall I add a FIXME when picking this up?
(If so, could you please suggest a text?)

 Thomas