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

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On 17.01.18 08:57, Heiko Carstens wrote:
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)

I'm doing that because the gisa requires "integral boundary" alignment that changes with a later patch when I introduce format-1.


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.

adding this to the definition does not allow to use sizeof() for the currently defined struct. Thus I would have to explicitly specify the
size... :(

But of course it is all naturally aligned anyway... ;)


Thanks
Michael

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