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 <[email protected]> > > > 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.
