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Re: [PATCH] pc-bios: s390x: Only set lowcore iplb address on list-direct

From: Janosch Frank
Subject: Re: [PATCH] pc-bios: s390x: Only set lowcore iplb address on list-directed IPL
Date: Wed, 19 Aug 2020 12:46:11 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0

On 8/19/20 11:45 AM, Cornelia Huck wrote:
> On Wed, 19 Aug 2020 11:32:34 +0200
> Janosch Frank <frankja@linux.ibm.com> wrote:
>> On 8/17/20 7:51 PM, Jason J. Herne wrote:
>>> On 8/17/20 12:30 PM, Cornelia Huck wrote:  
>>>> On Mon, 17 Aug 2020 10:17:34 -0400
>>>> "Jason J. Herne" <jjherne@linux.ibm.com> wrote:
>>>>> The POP states that the IPLB location is only written to 0x14 for
>>>>> list-directed IPL. Some operating systems expect 0x14 to not change on
>>>>> boot and will fail IPL if it does change.
>>>>> Fixes: 9bfc04f9ef6802fff0  
>>>> Should be
>>>> Fixes: 9bfc04f9ef68 ("pc-bios: s390x: Save iplb location in lowcore")
>>>>> Signed-off-by: Jason J. Herne <jjherne@linux.ibm.com>
>>>>> Reviewed-by: Janosch Frank <frankja@de.ibm.com>
>>>>> ---
>>>>>   pc-bios/s390-ccw/jump2ipl.c | 5 ++++-
>>>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>>>> diff --git a/pc-bios/s390-ccw/jump2ipl.c b/pc-bios/s390-ccw/jump2ipl.c
>>>>> index 767012bf0c..5e3e13f4b0 100644
>>>>> --- a/pc-bios/s390-ccw/jump2ipl.c
>>>>> +++ b/pc-bios/s390-ccw/jump2ipl.c
>>>>> @@ -33,7 +33,10 @@ void jump_to_IPL_code(uint64_t address)
>>>>>   {
>>>>>       /* store the subsystem information _after_ the bootmap was loaded */
>>>>>       write_subsystem_identification();
>>>>> -    write_iplb_location();
>>>>> +
>>>>> +    if (iplb.pbt != S390_IPL_TYPE_CCW) {
>>>>> +            write_iplb_location();
>>>>> +    }  
>>>> What happens for ipl types other than CCW and FCP? IOW, should that
>>>> rather be a positive check for S390_IPL_TYPE_FCP?
>>>>>       /* prevent unknown IPL types in the guest */
>>>>>       if (iplb.pbt == S390_IPL_TYPE_QEMU_SCSI) {  
>>> Based on my (admittedly limited) understanding of the architecture and
>>> code, I believe write_iplb_location() should be called at least for
>>> S390_IPL_TYPE_FCP but I'm not 100% sure on S390_IPL_TYPE_QEMU_SCSI.
>>> Perhaps Janosch has an idea?
>>> It was originally unconditional, and my new conditional excludes vfio
>>> CCW which is definitely a step in the right direction, in any case :).  
>> If I remember correctly the problem was that ZIPL used the IPLB lowcore
>> ptr without checking how it was booted (CCW or FCP). That was fixed in
>> mid of July by testing if diag308 gives back a config or not.
> So we have the problem that old zipl relies on the presence of a value
> that must not be there if you follow the architecture? Nasty.
> (Is it really "must not change" vs "don't expect anything here"? Not
> sure if I'm looking at the right part of the documentation.)

Well if the loaded program overwrites absolute 0x0, we shouldn't modify
it if we are not explicitly allowed to, no?

We already talked about saving the exception new addresses and restoring
them before jumping to the new kernel. I think we might need to go a
step further and use a non zero prefix for the bios to avoid any changes
to absolute 0x0.

However that wouldn't fix this dilemma.

>> So we might have a deadlock situation here which I need to think about
>> for a bit. I'm setting Viktor CC to get a bit more information about the
>> state of the zipl backports into the distros.
> Changing this is problematic unless everything we support as a guest is
> fixed. Does the guest go boom in a way that it is at least easy to
> figure out what went wrong? What breaks when the value continues to be
> set?
I think it goes into disabled wait because of the failed secure boot
verification or gets a PGM Addressing and then goes into disabled wait,
so no, not very user friendly.

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