On 27.02.20 10:47, Cornelia Huck wrote:
On Thu, 27 Feb 2020 10:10:31 +0100
Michael Mueller <mimu@xxxxxxxxxxxxx> wrote:
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
Maybe log something as well if it is off due to this kernel parameter?
There is also no message when the host does not support it.
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@xxxxxxxxxxxxx>
---
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.");
I probably would have inverted the logic (i.e. introduce a disable_gisa
parameter that is off by default), as you want KVM to use the gisa,
except in special circumstances.
Hm, in my opinion "use it := no" is more explicit and natural
than "don't use it := yes"
+
/*
* 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);
I assume we're fine with no gisa but a gib (i.e. doesn't hurt?)
The GIB is a host entity and is required for guests with GISA that want
to use AP with interruptions in guest.
KVM_EVENT(3, "vm 0x%pK created by pid %u", kvm, current->pid);
return 0;
Thanks a lot!
Michael
