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 --- Documentation/virtual/kvm/hypercalls.txt | 22 +++++++++++++++ arch/x86/kvm/x86.c | 43 +++++++++++++++++++++++++++++++ include/uapi/linux/kvm_para.h | 2 + 3 files changed, 67 insertions(+) Index: kvm.fifopriohc-submit/Documentation/virtual/kvm/hypercalls.txt =================================================================== --- kvm.fifopriohc-submit.orig/Documentation/virtual/kvm/hypercalls.txt +++ kvm.fifopriohc-submit/Documentation/virtual/kvm/hypercalls.txt @@ -121,3 +121,25 @@ compute the CLOCK_REALTIME for its clock Returns KVM_EOPNOTSUPP if the host does not use TSC clocksource, or if clock type is different than KVM_CLOCK_PAIRING_WALLCLOCK. + +6. KVM_HC_RT_PRIO +------------------------ +Architecture: x86 +Status: active +Purpose: Hypercall used to change qemu vcpu process -RT priority. + +Usage: Having a pCPU share a FIFO:1 vcpu and a QEMU emulator thread +can be problematic: especially if the vcpu busy-spins on memory waiting +for the QEMU emulator thread to write to, which leads to a hang +(because the FIFO:1 vcpu is never scheduled in favor of QEMU emulator +thread). +So this hypercall is supposed to be called by the guest when +the OS knows its not going to busy spin on memory thats +written by the emulator thread as above. + +a0: bit 0 contains enable bit, if 0 indicates that SCHED_OTHER +priority should be set for vcpu, if 1 indicates SCHED_FIFO +priority (the actual value for FIFO priority is decided +by the host). + + Index: kvm.fifopriohc-submit/include/uapi/linux/kvm_para.h =================================================================== --- kvm.fifopriohc-submit.orig/include/uapi/linux/kvm_para.h +++ kvm.fifopriohc-submit/include/uapi/linux/kvm_para.h @@ -15,6 +15,7 @@ #define KVM_E2BIG E2BIG #define KVM_EPERM EPERM #define KVM_EOPNOTSUPP 95 +#define KVM_EINVAL EINVAL #define KVM_HC_VAPIC_POLL_IRQ 1 #define KVM_HC_MMU_OP 2 @@ -25,6 +26,7 @@ #define KVM_HC_MIPS_EXIT_VM 7 #define KVM_HC_MIPS_CONSOLE_OUTPUT 8 #define KVM_HC_CLOCK_PAIRING 9 +#define KVM_HC_RT_PRIO 10 /* * hypercalls use architecture specific Index: kvm.fifopriohc-submit/arch/x86/kvm/x86.c =================================================================== --- kvm.fifopriohc-submit.orig/arch/x86/kvm/x86.c +++ kvm.fifopriohc-submit/arch/x86/kvm/x86.c @@ -66,6 +66,8 @@ #include #include #include +#include +#include #define CREATE_TRACE_POINTS #include "trace.h" @@ -6261,6 +6263,44 @@ void kvm_vcpu_deactivate_apicv(struct kv kvm_x86_ops->refresh_apicv_exec_ctrl(vcpu); } +static int convert_to_kvm_errcode(int error) +{ + switch (error) { + case -EPERM: + return -KVM_EPERM; + case -EINVAL: + default: + return -KVM_EINVAL; + } +} + +int kvm_pv_rt_prio(struct kvm_vcpu *vcpu, unsigned long a0) +{ + int ret; + bool enable; + struct sched_param param; + + memset(¶m, 0, sizeof(struct sched_param)); + param.sched_priority = vcpu->arch.rt_sched_priority; + + enable = a0 & 0x1; + + if (vcpu->arch.enable_rt_prio_hc == false) + return -KVM_EPERM; + + if (enable) { + ret = sched_setscheduler(current, SCHED_FIFO, ¶m); + } else { + param.sched_priority = 0; + ret = sched_setscheduler(current, SCHED_NORMAL, ¶m); + } + + if (ret) + ret = convert_to_kvm_errcode(ret); + + return ret; +} + int kvm_emulate_hypercall(struct kvm_vcpu *vcpu) { unsigned long nr, a0, a1, a2, a3, ret; @@ -6306,6 +6346,9 @@ int kvm_emulate_hypercall(struct kvm_vcp ret = kvm_pv_clock_pairing(vcpu, a0, a1); break; #endif + case KVM_HC_RT_PRIO: + ret = kvm_pv_rt_prio(vcpu, a0); + break; default: ret = -KVM_ENOSYS; break;