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Re: [PATCH V13 1/8] accel/kvm: Extract common KVM vCPU {creation,parking

From: Salil Mehta
Subject: Re: [PATCH V13 1/8] accel/kvm: Extract common KVM vCPU {creation,parking} code
Date: Mon, 8 Jul 2024 23:30:01 +0000
User-agent: Mozilla Thunderbird 
Hi Igor,

On 08/07/2024 13:32, Igor Mammedov wrote:

On Sat, 6 Jul 2024 15:43:01 +0000
Salil Mehta <salil.mehta@opnsrc.net> wrote:


Hi Igor,
Thanks for taking out time to review.

On Sat, Jul 6, 2024 at 1:12 PM Igor Mammedov <imammedo@redhat.com> wrote:


On Fri, 7 Jun 2024 12:56:42 +0100
Salil Mehta <salil.mehta@huawei.com> wrote:

KVM vCPU creation is done once during the vCPU realization when Qemu

vCPU thread

is spawned. This is common to all the architectures as of now.

Hot-unplug of vCPU results in destruction of the vCPU object in QOM but

the

corresponding KVM vCPU object in the Host KVM is not destroyed as KVM

doesn't

support vCPU removal. Therefore, its representative KVM vCPU

object/context in

Qemu is parked.

Refactor architecture common logic so that some APIs could be reused by

vCPU

Hotplug code of some architectures likes ARM, Loongson etc. Update

new/old APIs

with trace events. No functional change is intended here.

Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Tested-by: Vishnu Pajjuri <vishnu@os.amperecomputing.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Tested-by: Xianglai Li <lixianglai@loongson.cn>
Tested-by: Miguel Luis <miguel.luis@oracle.com>
Reviewed-by: Shaoqin Huang <shahuang@redhat.com>
Reviewed-by: Vishnu Pajjuri <vishnu@os.amperecomputing.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
---
  accel/kvm/kvm-all.c    | 95 ++++++++++++++++++++++++++++--------------
  accel/kvm/kvm-cpus.h   |  1 -
  accel/kvm/trace-events |  5 ++-
  include/sysemu/kvm.h   | 25 +++++++++++
  4 files changed, 92 insertions(+), 34 deletions(-)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index c0be9f5eed..8f9128bb92 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -340,14 +340,71 @@ err:
      return ret;
  }

+void kvm_park_vcpu(CPUState *cpu)
+{
+    struct KVMParkedVcpu *vcpu;
+
+    trace_kvm_park_vcpu(cpu->cpu_index, kvm_arch_vcpu_id(cpu));
+
+    vcpu = g_malloc0(sizeof(*vcpu));
+    vcpu->vcpu_id = kvm_arch_vcpu_id(cpu);
+    vcpu->kvm_fd = cpu->kvm_fd;
+    QLIST_INSERT_HEAD(&kvm_state->kvm_parked_vcpus, vcpu, node);
+}
+
+int kvm_unpark_vcpu(KVMState *s, unsigned long vcpu_id)
+{
+    struct KVMParkedVcpu *cpu;
+    int kvm_fd = -ENOENT;
+
+    QLIST_FOREACH(cpu, &s->kvm_parked_vcpus, node) {
+        if (cpu->vcpu_id == vcpu_id) {
+            QLIST_REMOVE(cpu, node);
+            kvm_fd = cpu->kvm_fd;
+            g_free(cpu);
+        }
+    }
+
+    trace_kvm_unpark_vcpu(vcpu_id, kvm_fd > 0 ? "unparked" : "not found

parked");

+
+    return kvm_fd;
+}
+
+int kvm_create_vcpu(CPUState *cpu)
+{
+    unsigned long vcpu_id = kvm_arch_vcpu_id(cpu);
+    KVMState *s = kvm_state;
+    int kvm_fd;
+
+    /* check if the KVM vCPU already exist but is parked */
+    kvm_fd = kvm_unpark_vcpu(s, vcpu_id);
+    if (kvm_fd < 0) {
+        /* vCPU not parked: create a new KVM vCPU */
+        kvm_fd = kvm_vm_ioctl(s, KVM_CREATE_VCPU, vcpu_id);
+        if (kvm_fd < 0) {
+            error_report("KVM_CREATE_VCPU IOCTL failed for vCPU %lu",

vcpu_id);

+            return kvm_fd;
+        }
+    }
+
+    cpu->kvm_fd = kvm_fd;
+    cpu->kvm_state = s;
+    cpu->vcpu_dirty = true;
+    cpu->dirty_pages = 0;
+    cpu->throttle_us_per_full = 0;
+
+    trace_kvm_create_vcpu(cpu->cpu_index, vcpu_id, kvm_fd);
+
+    return 0;
+}

Is there any reason why you are embedding/hiding kvm_state in new API
instead of passing it as argument (all callers have it defined, so why not
reuse that)?

It is a global variable and I don't think it is a usual practice to specify
the global variable
as an input parameter.

Ideally, global would be accessed once at API boundary entry
and the passed as an argument to functions it calls.
It makes it easier to follow as opposed to mixed access we have now,
which is harder to review since one has to check both
flavors (argument passed or directly accessed).

in this patch  kvm_init_vcpu() calls new kvm_create_vcpu()
and the former caches these global into 's' local variable,
so I'd reuse that local variable like kvm_get_vcpu() you are removing here did.

That is one perspective, but:
1. kvm_create_vcpu() will also be called externally from other contexts.
   It would be awkward to pass this variable from those non-local places
   where it would seem unnecessary.
2. If you look at other symmetrical functions like kvm_destroy_vcpu(),
   they also have a similar prototype.

I think it is about doing a slight trade-off. If you really believe this
change is necessary for us to proceed, I will make the adjustment.
However, please note that it will affect the IBM team as well.

Hi Nick,

I hope you are okay with this suggested change?

Thanks,
Salil

otherwise patch lgtm


May I request your Reviewed-by for this patch?

Thanks
Salil.



+
  static int do_kvm_destroy_vcpu(CPUState *cpu)
  {
      KVMState *s = kvm_state;
      long mmap_size;
-    struct KVMParkedVcpu *vcpu = NULL;
      int ret = 0;

-    trace_kvm_destroy_vcpu();
+    trace_kvm_destroy_vcpu(cpu->cpu_index, kvm_arch_vcpu_id(cpu));

      ret = kvm_arch_destroy_vcpu(cpu);
      if (ret < 0) {
@@ -373,10 +430,7 @@ static int do_kvm_destroy_vcpu(CPUState *cpu)
          }
      }

-    vcpu = g_malloc0(sizeof(*vcpu));
-    vcpu->vcpu_id = kvm_arch_vcpu_id(cpu);
-    vcpu->kvm_fd = cpu->kvm_fd;
-    QLIST_INSERT_HEAD(&kvm_state->kvm_parked_vcpus, vcpu, node);
+    kvm_park_vcpu(cpu);
  err:
      return ret;
  }
@@ -389,24 +443,6 @@ void kvm_destroy_vcpu(CPUState *cpu)
      }
  }

-static int kvm_get_vcpu(KVMState *s, unsigned long vcpu_id)
-{
-    struct KVMParkedVcpu *cpu;
-
-    QLIST_FOREACH(cpu, &s->kvm_parked_vcpus, node) {
-        if (cpu->vcpu_id == vcpu_id) {
-            int kvm_fd;
-
-            QLIST_REMOVE(cpu, node);
-            kvm_fd = cpu->kvm_fd;
-            g_free(cpu);
-            return kvm_fd;
-        }
-    }
-
-    return kvm_vm_ioctl(s, KVM_CREATE_VCPU, (void *)vcpu_id);
-}
-
  int kvm_init_vcpu(CPUState *cpu, Error **errp)
  {
      KVMState *s = kvm_state;
@@ -415,19 +451,14 @@ int kvm_init_vcpu(CPUState *cpu, Error **errp)

      trace_kvm_init_vcpu(cpu->cpu_index, kvm_arch_vcpu_id(cpu));

-    ret = kvm_get_vcpu(s, kvm_arch_vcpu_id(cpu));
+    ret = kvm_create_vcpu(cpu);
      if (ret < 0) {
-        error_setg_errno(errp, -ret, "kvm_init_vcpu: kvm_get_vcpu

failed (%lu)",

+        error_setg_errno(errp, -ret,
+                         "kvm_init_vcpu: kvm_create_vcpu failed (%lu)",
                           kvm_arch_vcpu_id(cpu));
          goto err;
      }

-    cpu->kvm_fd = ret;
-    cpu->kvm_state = s;
-    cpu->vcpu_dirty = true;
-    cpu->dirty_pages = 0;
-    cpu->throttle_us_per_full = 0;
-
      mmap_size = kvm_ioctl(s, KVM_GET_VCPU_MMAP_SIZE, 0);
      if (mmap_size < 0) {
          ret = mmap_size;
diff --git a/accel/kvm/kvm-cpus.h b/accel/kvm/kvm-cpus.h
index ca40add32c..171b22fd29 100644
--- a/accel/kvm/kvm-cpus.h
+++ b/accel/kvm/kvm-cpus.h
@@ -22,5 +22,4 @@ bool kvm_supports_guest_debug(void);
  int kvm_insert_breakpoint(CPUState *cpu, int type, vaddr addr, vaddr

len);

  int kvm_remove_breakpoint(CPUState *cpu, int type, vaddr addr, vaddr

len);

  void kvm_remove_all_breakpoints(CPUState *cpu);
-
  #endif /* KVM_CPUS_H */
diff --git a/accel/kvm/trace-events b/accel/kvm/trace-events
index 681ccb667d..37626c1ac5 100644
--- a/accel/kvm/trace-events
+++ b/accel/kvm/trace-events
@@ -9,6 +9,10 @@ kvm_device_ioctl(int fd, int type, void *arg) "dev fd

%d, type 0x%x, arg %p"

  kvm_failed_reg_get(uint64_t id, const char *msg) "Warning: Unable to

retrieve ONEREG %" PRIu64 " from KVM: %s"

  kvm_failed_reg_set(uint64_t id, const char *msg) "Warning: Unable to

set ONEREG %" PRIu64 " to KVM: %s"

  kvm_init_vcpu(int cpu_index, unsigned long arch_cpu_id) "index: %d id:

%lu"

+kvm_create_vcpu(int cpu_index, unsigned long arch_cpu_id, int kvm_fd)

"index: %d, id: %lu, kvm fd: %d"

+kvm_destroy_vcpu(int cpu_index, unsigned long arch_cpu_id) "index: %d

id: %lu"

+kvm_park_vcpu(int cpu_index, unsigned long arch_cpu_id) "index: %d id:

%lu"

+kvm_unpark_vcpu(unsigned long arch_cpu_id, const char *msg) "id: %lu %s"
  kvm_irqchip_commit_routes(void) ""
  kvm_irqchip_add_msi_route(char *name, int vector, int virq) "dev %s

vector %d virq %d"

  kvm_irqchip_update_msi_route(int virq) "Updating MSI route virq=%d"
@@ -25,7 +29,6 @@ kvm_dirty_ring_reaper(const char *s) "%s"
  kvm_dirty_ring_reap(uint64_t count, int64_t t) "reaped %"PRIu64" pages

(took %"PRIi64" us)"

  kvm_dirty_ring_reaper_kick(const char *reason) "%s"
  kvm_dirty_ring_flush(int finished) "%d"
-kvm_destroy_vcpu(void) ""
  kvm_failed_get_vcpu_mmap_size(void) ""
  kvm_cpu_exec(void) ""
  kvm_interrupt_exit_request(void) ""
diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
index c31d9c7356..c4a914b3d8 100644
--- a/include/sysemu/kvm.h
+++ b/include/sysemu/kvm.h
@@ -313,6 +313,31 @@ int kvm_create_device(KVMState *s, uint64_t type,

bool test);

   */
  bool kvm_device_supported(int vmfd, uint64_t type);

+/**
+ * kvm_create_vcpu - Gets a parked KVM vCPU or creates a KVM vCPU
+ * @cpu: QOM CPUState object for which KVM vCPU has to be

fetched/created.

+ *
+ * @returns: 0 when success, errno (<0) when failed.
+ */
+int kvm_create_vcpu(CPUState *cpu);
+
+/**
+ * kvm_park_vcpu - Park QEMU KVM vCPU context
+ * @cpu: QOM CPUState object for which QEMU KVM vCPU context has to be

parked.

+ *
+ * @returns: none
+ */
+void kvm_park_vcpu(CPUState *cpu);
+
+/**
+ * kvm_unpark_vcpu - unpark QEMU KVM vCPU context
+ * @s: KVM State
+ * @vcpu_id: Architecture vCPU ID of the parked vCPU
+ *
+ * @returns: KVM fd
+ */
+int kvm_unpark_vcpu(KVMState *s, unsigned long vcpu_id);
+
  /* Arch specific hooks */

  extern const KVMCapabilityInfo kvm_arch_required_capabilities[];
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