On 27.02.20 13:48, Cornelia Huck wrote:
On Thu, 27 Feb 2020 13:27:10 +0100
Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
On 27.02.20 10:10, Michael Mueller 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
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>
Looks good to me. Regarding the other comments, I think allowing for dynamic changes
and keeping use_gisa vs disable_gisa makes sense. So I would think that the patch
as is makes sense.
use_gisa vs disable_gisa is more a personal preference; I don't mind
keeping it as use_gisa.
The only question is: shall we set use_gisa to 0 when the machine does not support
it (e.g. VSIE?) and then also forbid setting it to 1? Could be overkill.
I don't think you should try to overload a debug knob like that; it's
now simple enough, adding more code also adds to the potential for
errors.
---
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.");
Especially as the description explicitly says "if the host supports it"
-- that's good enough for a new knob.
+
/*
* 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;
Reviewed-by: Cornelia Huck <cohuck@xxxxxxxxxx>
Thanks!
