KVM: s390: add capability indicating COW support

Currently qemu/kvm on s390 uses a guest mapping that does not
allow the guest backing page table to be write-protected to
support older systems. On those older systems a host write
protection fault will be delivered to the guest.

Newer systems allow to write-protect the guest backing memory
and let the fault be delivered to the host, thus allowing COW.

Use a capability bit to tell qemu if that is possible.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/s390/include/asm/sclp.h

@@ -48,6 +48,7 @@ int sclp_cpu_deconfigure(u8 cpu);
 void sclp_facilities_detect(void);
 unsigned long long sclp_get_rnmax(void);
 unsigned long long sclp_get_rzm(void);
+u8 sclp_get_fac85(void);
 int sclp_sdias_blk_count(void);
 int sclp_sdias_copy(void *dest, int blk_num, int nr_blks);
 int sclp_chp_configure(struct chp_id chpid);

arch/s390/kvm/kvm-s390.c

@@ -28,6 +28,7 @@
 #include <asm/pgtable.h>
 #include <asm/nmi.h>
 #include <asm/switch_to.h>
+#include <asm/sclp.h>
 #include "kvm-s390.h"
 #include "gaccess.h"
 
@@ -140,6 +141,9 @@ int kvm_dev_ioctl_check_extension(long ext)
 	case KVM_CAP_MAX_VCPUS:
 		r = KVM_MAX_VCPUS;
 		break;
+	case KVM_CAP_S390_COW:
+		r = sclp_get_fac85() & 0x2;
+		break;
 	default:
 		r = 0;
 	}

drivers/s390/char/sclp_cmd.c

@@ -17,6 +17,7 @@
 #include <linux/mm.h>
 #include <linux/mmzone.h>
 #include <linux/memory.h>
+#include <linux/module.h>
 #include <linux/platform_device.h>
 #include <asm/chpid.h>
 #include <asm/sclp.h>
@@ -38,7 +39,8 @@ struct read_info_sccb {
 	u64	facilities;		/* 48-55 */
 	u8	_reserved2[84 - 56];	/* 56-83 */
 	u8	fac84;			/* 84 */
-	u8	_reserved3[91 - 85];	/* 85-90 */
+	u8	fac85;			/* 85 */
+	u8	_reserved3[91 - 86];	/* 86-90 */
 	u8	flags;			/* 91 */
 	u8	_reserved4[100 - 92];	/* 92-99 */
 	u32	rnsize2;		/* 100-103 */
@@ -51,6 +53,7 @@ static int __initdata early_read_info_sccb_valid;
 
 u64 sclp_facilities;
 static u8 sclp_fac84;
+static u8 sclp_fac85;
 static unsigned long long rzm;
 static unsigned long long rnmax;
 
@@ -112,6 +115,7 @@ void __init sclp_facilities_detect(void)
 	sccb = &early_read_info_sccb;
 	sclp_facilities = sccb->facilities;
 	sclp_fac84 = sccb->fac84;
+	sclp_fac85 = sccb->fac85;
 	rnmax = sccb->rnmax ? sccb->rnmax : sccb->rnmax2;
 	rzm = sccb->rnsize ? sccb->rnsize : sccb->rnsize2;
 	rzm <<= 20;
@@ -127,6 +131,12 @@ unsigned long long sclp_get_rzm(void)
 	return rzm;
 }
 
+u8 sclp_get_fac85(void)
+{
+	return sclp_fac85;
+}
+EXPORT_SYMBOL_GPL(sclp_get_fac85);
+
 /*
  * This function will be called after sclp_facilities_detect(), which gets
  * called from early.c code. Therefore the sccb should have valid contents.

include/linux/kvm.h

@@ -616,6 +616,7 @@ struct kvm_ppc_smmu_info {
 #define KVM_CAP_KVMCLOCK_CTRL 76
 #define KVM_CAP_SIGNAL_MSI 77
 #define KVM_CAP_PPC_GET_SMMU_INFO 78
+#define KVM_CAP_S390_COW 79
 
 #ifdef KVM_CAP_IRQ_ROUTING