[PATCH] KVM: s390: introduce module parameter kvm.use_gisa

Michael Mueller
2020-02-27T09:10:44Z

The boolean module parameter "kvm.use_gisa" controls if newly
created guests will use the GISA facility if provided by the
host system. The default is yes.

# cat /sys/module/kvm/parameters/use_gisa
Y

The parameter can be changed on the fly.

# echo N > /sys/module/kvm/parameters/use_gisa

Already running guests are not affected by this change.

The kvm s390 debug feature shows if a guest is running with GISA.

# grep gisa /sys/kernel/debug/s390dbf/kvm-$pid/sprintf
00 01582725059:843303 3 - 08 00000000e119bc01 gisa 0x00000000c9ac2642 initialized
00 01582725059:903840 3 - 11 000000004391ee22 00[0000000000000000-0000000000000000]: AIV gisa format-1 enabled for cpu 000
...
00 01582725059:916847 3 - 08 0000000094fff572 gisa 0x00000000c9ac2642 cleared

In general, that value should not be changed as the GISA facility
enhances interruption delivery performance.

A reason to switch the GISA facility off might be a performance
comparison run or debugging.

Signed-off-by: Michael Mueller <mimu@linux.ibm.com>
---
arch/s390/kvm/kvm-s390.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index d7ff30e45589..5c2081488024 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -184,6 +184,11 @@ static u8 halt_poll_max_steal = 10;
module_param(halt_poll_max_steal, byte, 0644);
MODULE_PARM_DESC(halt_poll_max_steal, "Maximum percentage of steal time to allow polling");

+/* if set to true, the GISA will be initialized and used if available */
+static bool use_gisa = true;
+module_param(use_gisa, bool, 0644);
+MODULE_PARM_DESC(use_gisa, "Use the GISA if the host supports it.");
+
/*
* For now we handle at most 16 double words as this is what the s390 base
* kernel handles and stores in the prefix page. If we ever need to go beyond
@@ -2504,7 +2509,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
kvm->arch.use_skf = sclp.has_skey;
spin_lock_init(&kvm->arch.start_stop_lock);
kvm_s390_vsie_init(kvm);
- kvm_s390_gisa_init(kvm);
+ if (use_gisa)
+ kvm_s390_gisa_init(kvm);
KVM_EVENT(3, "vm 0x%pK created by pid %u", kvm, current->pid);

return 0;
--
2.17.1