Re: [PATCH v3 1/9] s390/alternatives: Add new ALT_TYPE_PERCPU type

[David Laight <david.laight.linux@gmail.com>]

On Wed, 20 May 2026 11:22:35 +0200
Heiko Carstens <hca@linux.ibm.com> wrote:

> The upcoming percpu section code uses two mviy instructions to guard the
> beginning and end of a percpu code section.
> 
> The first mviy instruction writes the register number, which contains the
> percpu address to lowcore. This indicates both the beginning of a percpu
> code section and which register contains the percpu address.
> 
> During compile time the mviy instruction is generated in a way that its
> base register contains the percpu register, and the immediate field is
> zero. This needs to be patched so that the base register is zero, and the
> immediate field contains the register number. For example
> 
>   101424:       eb 00 23 c0 00 52       mviy    960(%r2),0
> 
> needs to be patched to
> 
>   101424:       eb 20 03 c0 00 52       mviy    960(%r0),2

I'm sure it is possible get the preprocessor to extract the register number
for you.
The exception table logic almost certainly already does it.
(The x86 version certainly does - and that is far less trivial.)

-- David


> 
> Provide a new ALT_TYPE_PERCPU alternative type which handles this specific
> instruction patching. In addition it also handles the relocated lowcore
> case, where the displacement of the mviy instruction has a different value.