Re: [PATCH 02/12] KVM: s390: define GISA format-0 data structure

[Heiko Carstens <heiko.carstens@de.ibm.com>]

On Tue, Jan 16, 2018 at 09:25:12PM +0100, David Hildenbrand wrote:
> 
> > +struct kvm_s390_gisa {
> > +	u32 next_alert;
> > +	u8 ipm;
> > +	u8 reserved01;
> > +	u8:6;
> > +	u8 g:1;
> > +	u8 c:1;
> > +	u8 iam;
> > +	u8 reserved02[4];
> > +	u32 airq_count;
> > +};
> > +
> >  /*
> > - * sie_page2 has to be allocated as DMA because fac_list and crycb need
> > - * 31bit addresses in the sie control block.
> > + * sie_page2 has to be allocated as DMA because fac_list, crycb and
> > + * gisa need 31bit addresses in the sie control block.
> >   */
> >  struct sie_page2 {
> >  	__u64 fac_list[S390_ARCH_FAC_LIST_SIZE_U64];	/* 0x0000 */
> >  	struct kvm_s390_crypto_cb crycb;		/* 0x0800 */
> > -	u8 reserved900[0x1000 - 0x900];			/* 0x0900 */
> > +	struct kvm_s390_gisa gisa __aligned(sizeof(struct kvm_s390_gisa)); /* 0x0900 */
> 
> can we instead specify sie_page2 as packed if really needed? (I can see
> that we have a BUILD_BUG_ON below, which is nice)
> 
> (alignment within well defined data structures looks strange)

The alignment, if needed, should go to the definition of struct
kvm_s390_gisa above. The structure needs at least an alignment of eight
bytes since bitops are used to modify ipm (in a later patch), by using a
(long *) cast to this structure.
But of course it is all naturally aligned anyway... ;)