On 2/19/21 5:09 PM, Cornelia Huck wrote:
On Mon, 15 Feb 2021 17:06:50 +0100
Pierre Morel <pmorel@xxxxxxxxxxxxx> wrote:
Make $SUBJECT
"KVM: s390: diag9c (directed yield) forwarding" ?
When we receive intercept a DIAG_9C from the guest we verify
Either 'receive' or 'intercept', I guess :)
right.
...snip...
+DIAG 'X'9C forwarding
++++++++++++++++++++++
+
+Under KVM, the guest operating system may send a DIAGNOSE code 'X'9C to
+the host when it fails to acquire a spinlock for a virtual CPU
+and detects that the host CPU on which the virtual guest CPU owner is
+assigned to is not running to try to get this host CPU running and
+consequently the guest virtual CPU running and freeing the lock.
What about:
"The guest may send a DIAGNOSE 0x9c in order to yield to a certain
other vcpu. An example is a Linux guest that tries to yield to the vcpu
that is currently holding a spinlock, but not running."
Yes, thanks.
+
+However, on the logical partition the real CPU on which the previously
+targeted host CPU is assign may itself not be running.
"However, on the host the real cpu backing the vcpu may itself not be
running."
Yes, better too, thanks.
+By forwarding the DIAGNOSE code 'X'9C, initially sent by the guest,
+from the host to LPAR hypervisor, this one will hopefully schedule
+the host CPU which will let KVM run the target guest CPU.
"Forwarding the DIAGNOSE 0x9c initially sent by the guest to yield to
the backing cpu will hopefully cause that cpu, and thus subsequently
the guest's vcpu, to be scheduled."
yes.
[I don't think we should explicitly talk about LPAR here, as the same
should apply if we are running second-or-deeper level, right?]
yes right.
+
+diag9c_forwarding_hz
+ KVM kernel parameter allowing to specify the maximum number of DIAGNOSE
+ 'X'9C forwarding per second in the purpose of avoiding a DIAGNOSE 'X'9C
+ forwarding storm.
I think 0x9c is the more common way to write the hex code.
yes it is the purpose was to keep it consistent with the existing
documentation
Also,
"A value of 0 turns the forwarding off" ?
yes, I can add this explicitly.
(...)
diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
index 27c763014114..15e207a671fd 100644
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -422,6 +422,7 @@ void notrace smp_yield_cpu(int cpu)
asm volatile("diag %0,0,0x9c"
: : "d" (pcpu_devices[cpu].address));
}
+EXPORT_SYMBOL(smp_yield_cpu);
EXPORT_SYMBOL_GPL?
OK, clear.
/*
* Send cpus emergency shutdown signal. This gives the cpus the
(...)
@@ -190,6 +191,11 @@ static bool use_gisa = true;
module_param(use_gisa, bool, 0644);
MODULE_PARM_DESC(use_gisa, "Use the GISA if the host supports it.");
+/* maximum diag9c forwarding per second */
+unsigned int diag9c_forwarding_hz;
+module_param(diag9c_forwarding_hz, uint, 0644);
+MODULE_PARM_DESC(diag9c_forwarding_hz, "Maximum diag9c forwarding per second");
Maybe also add "(0 to turn off forwarding)" here?
OK, I will add it.
Thanks for the comments,
Regards,
Pierre
--
Pierre Morel
IBM Lab Boeblingen
