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

From: Jason J. Herne
Subject: Re: [PATCH] pc-bios: s390x: Only set lowcore iplb address on list-directed IPL
Date: Tue, 25 Aug 2020 10:45:22 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.0

On 8/25/20 7:38 AM, Thomas Huth wrote:
On 19/08/2020 12.46, Janosch Frank wrote:
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_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.

Sorry, I'm just back from summer vacation ... so what's the conclusion
for Jason's patch here? Should it be included as-is now or do we rather
neeed another rework here instead?


After some discussion we've decided to keep this internal for now. As it turns out, Janosch plans on making some changes here that will remove the need for this fix. So we'll just use it internally for our own testing purposes until that time.

Sorry for the noise :)

-- Jason J. Herne (jjherne@linux.ibm.com)

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