KVM: x86: Run PIT work in own kthread

We can't run PIT IRQ injection work in the interrupt context of the host
timer. This would allow the user to influence the handler complexity by
asking for a broadcast to a large number of VCPUs. Therefore, this work
was pushed into workqueue context in 9d244caf2e. However, this prevents
prioritizing the PIT injection over other task as workqueues share
kernel threads.

This replaces the workqueue with a kthread worker and gives that thread
a name in the format "kvm-pit/<owner-process-pid>". That allows to
identify and adjust the kthread priority according to the VM process
parameters.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index a7cb93cb215..eb62761b768 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -1810,6 +1810,14 @@ Valid flags are:
 
 #define KVM_PIT_SPEAKER_DUMMY     1 /* emulate speaker port stub */
 
+PIT timer interrupts may use a per-VM kernel thread for injection. If it
+exists, this thread will have a name of the following pattern:
+
+kvm-pit/<owner-process-pid>
+
+When running a guest with elevated priorities, the scheduling parameters of
+this thread may have to be adjusted accordingly.
+
 This IOCTL replaces the obsolete KVM_CREATE_PIT.