Re: [PATCH v2 12/12] s390/preempt: Enable HAS_SEPARATE_PREEMPT_RESCHED_BITS

[Heiko Carstens <hca@linux.ibm.com>]

On Thu, May 28, 2026 at 07:41:24AM -0700, Boqun Feng wrote:
> On Thu, May 28, 2026 at 12:53:25PM +0200, Peter Zijlstra wrote:
> > On Tue, May 26, 2026 at 08:21:48AM -0700, Boqun Feng wrote:
> > > From: Heiko Carstens <hca@linux.ibm.com>
> > > diff --git a/arch/s390/include/asm/lowcore.h b/arch/s390/include/asm/lowcore.h
> > > index 50ffe75adeb4..0974ab278169 100644
> > > --- a/arch/s390/include/asm/lowcore.h
> > > +++ b/arch/s390/include/asm/lowcore.h
> > > @@ -160,10 +160,15 @@ struct lowcore {
> > >  	/* SMP info area */
> > >  	__u32	cpu_nr;				/* 0x03a0 */
> > >  	__u32	softirq_pending;		/* 0x03a4 */
> > > -	__s32	preempt_count;			/* 0x03a8 */
> > > -	__u32	spinlock_lockval;		/* 0x03ac */
> > > -	__u32	spinlock_index;			/* 0x03b0 */
> > > -	__u8	pad_0x03b4[0x03b8-0x03b4];	/* 0x03b4 */
> > > +	union {
Ä> > > +		struct {
> > > +			__u32	need_resched;	/* 0x03a8 */
> > > +			__u32	count;		/* 0x03ac */
> > > +		} preempt;
> > > +		__u64	preempt_count;		/* 0x03a8 */
> > > +	};
> > 
> > I'm a little confused by this union; afaict it isn't actually used.
> 
> (TIL: s390 is big endian)
> 
> In preempt_count() the union is used for reading the lower 32bits in an
> asm block.

Yes, it is used there to read only the lower 32 bits (count) in order
to avoid masking out the higher 32 bits, which would be required with
a 64 bit load.