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

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On 27.02.20 10:26, David Hildenbrand 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>
---
  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);

Looks sane to me. gi->origin will remain NULL and act like
css_general_characteristics.aiv wouldn't be around.

right


I assume initializing the gib is fine, and having some guests use it and
others not?

Is fine as well.


I do wonder if it would be even clearer/cleaner to not allow to change
this property on the fly, and to also not init the gib if disabled.

If you want to perform performance tests, simply unload+reload KVM.

That would work if kvm is build as module but not in-kernel, then
a reboot would be required with kvm.use_gisa=Y/N

I tend to leave it as it is.

Thanks,
Michael






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