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Subject[patch 2/3] KVM: x86: KVM_HC_RT_PRIO hypercall (host-side)
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@redhat.com>

---
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 <asm/pvclock.h>
#include <asm/div64.h>
#include <asm/irq_remapping.h>
+#include <uapi/linux/sched/types.h>
+#include <uapi/linux/sched.h>

#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(&param, 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, &param);
+ } else {
+ param.sched_priority = 0;
+ ret = sched_setscheduler(current, SCHED_NORMAL, &param);
+ }
+
+ 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;

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