[patch 0/3] KVM KVM_HC_RT_PRIO hypercall support

[Marcelo Tosatti]

When executing guest vcpu-0 with FIFO:1 priority, which is necessary to
deal with the following situation:

VCPU-0 (housekeeping VCPU)              VCPU-1 (realtime VCPU)

raw_spin_lock(A)
interrupted, schedule task T-1          raw_spin_lock(A) (spin)

raw_spin_unlock(A)

Certain operations must interrupt guest vcpu-0 (see trace below).

To fix this issue, only change guest vcpu-0 to FIFO priority
on spinlock critical sections (see patch).

Hang trace
==========

Without FIFO priority:

qemu-kvm-6705  [002] ....1.. 767785.648964: kvm_exit: reason IO_INSTRUCTION rip 0xe8fe info 1f00039 0
qemu-kvm-6705  [002] ....1.. 767785.648965: kvm_exit: reason IO_INSTRUCTION rip 0xe911 info 3f60008 0
qemu-kvm-6705  [002] ....1.. 767785.648968: kvm_exit: reason IO_INSTRUCTION rip 0x8984 info 608000b 0
qemu-kvm-6705  [002] ....1.. 767785.648971: kvm_exit: reason IO_INSTRUCTION rip 0xb313 info 1f70008 0
qemu-kvm-6705  [002] ....1.. 767785.648974: kvm_exit: reason IO_INSTRUCTION rip 0xb514 info 3f60000 0
qemu-kvm-6705  [002] ....1.. 767785.648977: kvm_exit: reason PENDING_INTERRUPT rip 0x8052 info 0 0
qemu-kvm-6705  [002] ....1.. 767785.648980: kvm_exit: reason IO_INSTRUCTION rip 0xeee6 info 200040 0
qemu-kvm-6705  [002] ....1.. 767785.648999: kvm_exit: reason EPT_MISCONFIG rip 0x2120 info 0 0

With FIFO priority:

qemu-kvm-7636  [002] ....1.. 768218.205065: kvm_exit: reason IO_INSTRUCTION rip 0xb313 info 1f70008 0
qemu-kvm-7636  [002] ....1.. 768218.205068: kvm_exit: reason IO_INSTRUCTION rip 0x8984 info 608000b 0
qemu-kvm-7636  [002] ....1.. 768218.205071: kvm_exit: reason IO_INSTRUCTION rip 0xb313 info 1f70008 0
qemu-kvm-7636  [002] ....1.. 768218.205074: kvm_exit: reason IO_INSTRUCTION rip 0x8984 info 608000b 0
qemu-kvm-7636  [002] ....1.. 768218.205077: kvm_exit: reason IO_INSTRUCTION rip 0xb313 info 1f70008 0
..

Performance numbers (kernel compilation with make -j2)
======================================================

With hypercall: 4:40.  (make -j2)
Without hypercall: 3:38.  (make -j2)

Note for NFV workloads spinlock performance is not relevant
since DPDK should not enter the kernel (and housekeeping vcpu
performance is far from a key factor).

Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>