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

[Janosch Frank]

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

> 
>>
>>>
>>>>
>>>> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
>>>> ---
>>>>  pc-bios/s390-ccw/start.S | 10 ++++++++--
>>>>  1 file changed, 8 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/pc-bios/s390-ccw/start.S b/pc-bios/s390-ccw/start.S
>>>> index 01c4c21b26..b0fcb918cc 100644
>>>> --- a/pc-bios/s390-ccw/start.S
>>>> +++ b/pc-bios/s390-ccw/start.S
>>>> @@ -64,7 +64,10 @@ consume_sclp_int:
>>>>          stctg   %c0,%c0,0(%r15)
>>>>          oi      6(%r15),0x2
>>>>          lctlg   %c0,%c0,0(%r15)
>>>> -        /* prepare external call handler */
>>>> +        /*
>>>> +         * Prepare external new PSW as it might have been overwritten
>>>> +         * by a loaded blob
>>>> +         */
>>>>          larl %r1, external_new_code
>>>>          stg %r1, 0x1b8
>>>>          larl %r1, external_new_mask
>>>> @@ -84,7 +87,10 @@ consume_io_int:
>>>>          stctg %c6,%c6,0(%r15)
>>>>          oi    4(%r15), 0xff
>>>>          lctlg %c6,%c6,0(%r15)
>>>> -        /* prepare i/o call handler */
>>>> +        /*
>>>> +         * Prepare i/o new PSW as it might have been overwritten
>>>> +         * by a loaded blob
>>>> +         */
>>>>          larl  %r1, io_new_code
>>>>          stg   %r1, 0x1f8
>>>>          larl  %r1, io_new_mask
>>>>
>>
>>

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