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Re: [PATCH v2 2/2] pc-bios: s390x: Clear out leftover S390EP string

From: Thomas Huth
Subject: Re: [PATCH v2 2/2] pc-bios: s390x: Clear out leftover S390EP string
Date: Mon, 23 Nov 2020 09:12:02 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0

On 23/11/2020 09.07, Christian Borntraeger wrote:
> On 23.11.20 09:05, Thomas Huth wrote:
>> On 23/11/2020 08.39, Christian Borntraeger wrote:
>>> On 20.11.20 17:01, Eric Farman wrote:
>>>> A Linux binary will have the string "S390EP" at address 0x10008,
>>>> which is important in getting the guest up off the ground. In the
>>>> case of a reboot (specifically chreipl going to a new device),
>>>> we should defer to the PSW at address zero for the new config,
>>>> which will re-write "S390EP" from the new image.
>>>> Let's clear it out at this point so that a reipl to, say, a DASD
>>>> passthrough device drives the IPL path from scratch without disrupting
>>>> disrupting the order of operations for other boots.
>>>> Rather than hardcoding the address of this magic (again), let's
>>>> define it somewhere so that the two users are visibly related.
>>> Hmmm, this might have side effects, e.g. if you do something like a kdump
>>> or kexec to a non-Linux binary that happens to have code at 0x10008, no?
>> Do these scenarios really go through the s390-ccw bios again, or do they
>> rather bypass the bios and jump directly into the new kernel?
> Right they jump directly into the new kernel. So this patch could actually
> be "good enough" for 5.2 as is?

I think it should be fine, yes. I'll give it a try with my usual s390-ccw
bios tests, and if there are no regressions (and no other objections on the
mailing list here), I'll prepare a pull request.


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