Re: [Qemu-devel] [PATCH] [KVM] Needless to update msi route when only msi-x entry "control" section changed

[Zhanghaoyu (A)]

>> >> With regard to old version linux guest(e.g., rhel-5.5), in ISR 
>> >> processing, mask and unmask msi-x vector every time, which result in 
>> >> VMEXIT, then QEMU will invoke kvm_irqchip_update_msi_route() to ask KVM 
>> >> hypervisor to update the VM irq routing table. In KVM hypervisor, 
>> >> synchronizing RCU needed after updating routing table, so much time 
>> >> consumed for waiting in wait_rcu_gp(). So CPU usage in VM is so high, 
>> >> while from the view of host, VM's total CPU usage is so low. 
>> >> Masking/unmasking msi-x vector only set msi-x entry "control" section, 
>> >> needless to update VM irq routing table.
>> >> 
>> >> Signed-off-by: Zhang Haoyu <address@hidden>
>> >> Signed-off-by: Huang Weidong <address@hidden>
>> >> Signed-off-by: Qin Chuanyu <address@hidden>
>> >> ---
>> >> hw/i386/kvm/pci-assign.c | 3 +++
>> >> 1 files changed, 3 insertions(+)
>> >> 
>> >> --- a/hw/i386/kvm/pci-assign.c  2013-05-04 15:53:18.000000000 +0800
>> >> +++ b/hw/i386/kvm/pci-assign.c  2013-05-04 15:50:46.000000000 +0800
>> >> @@ -1576,6 +1576,8 @@ static void assigned_dev_msix_mmio_write
>> >>                  MSIMessage msg;
>> >>                  int ret;
>> >> 
>> >> +                /* Needless to update msi route when only msi-x entry 
>> >> "control" section changed */
>> >> +                if ((addr & (PCI_MSIX_ENTRY_SIZE - 1)) != 
>> >> + PCI_MSIX_ENTRY_VECTOR_CTRL){
>> >>                  msg.address = entry->addr_lo |
>> >>                      ((uint64_t)entry->addr_hi << 32);
>> >>                  msg.data = entry->data; @@ -1585,6 +1587,7 @@ 
>> >> static void assigned_dev_msix_mmio_write
>> >>                  if (ret) {
>> >>                      error_report("Error updating irq routing entry 
>> >> (%d)", ret);
>> >>                  }
>> >> +                }
>> >>              }
>> >>          }
>> >>      }
>> >> 
>> >> Thanks,
>> >> Zhang Haoyu
>> >
>> >
>> >If guest wants to update the vector, it does it like this:
>> >mask
>> >update
>> >unmask
>> >and it looks like the only point where we update the vector is on unmask, 
>> >so this patch will mean we don't update the vector ever.
>> >
>> >I'm not sure this combination (old guest + legacy device assignment
>> >framework) is worth optimizing. Can you try VFIO instead?
>> >
>> >But if it is, the right way to do this is probably along the lines of the 
>> >below patch. Want to try it out?
>> >
>> >diff --git a/kvm-all.c b/kvm-all.c
>> >index 2d92721..afe2327 100644
>> >--- a/kvm-all.c
>> >+++ b/kvm-all.c
>> >@@ -1006,6 +1006,11 @@ static int kvm_update_routing_entry(KVMState *s,
>> >             continue;
>> >         }
>> > 
>> >+        if (entry->type == new_entry->type &&
>> >+            entry->flags == new_entry->flags &&
>> >+            entry->u == new_entry->u) {
>> >+            return 0;
>> >+        }
>> >         entry->type = new_entry->type;
>> >         entry->flags = new_entry->flags;
>> >         entry->u = new_entry->u;
>> >
>> 
>> union type cannot be directly compared, I tried out below patch 
>> instead,
>> --- a/kvm-all.c 2013-05-06 09:56:38.000000000 +0800
>> +++ b/kvm-all.c 2013-05-06 09:56:45.000000000 +0800
>> @@ -1008,6 +1008,12 @@ static int kvm_update_routing_entry(KVMS
>>              continue;
>>          }
>> 
>> +        if (entry->type == new_entry->type &&
>> +            entry->flags == new_entry->flags &&
>> +            !memcmp(&entry->u, &new_entry->u, sizeof(entry->u))) {
>> +            return 0;
>> +        }
>> +
>>          entry->type = new_entry->type;
>>          entry->flags = new_entry->flags;
>>          entry->u = new_entry->u;
>> 
>> MST's patch is more universal than my first patch fixed in 
>> assigned_dev_msix_mmio_write().
>> On the case that the msix entry's other section but "control" section is set 
>> to the identical value with old entry's, MST's patch also works.
>> MST's patch also works on the non-passthrough scenario.
>
>Any numbers for either case?
>
I'm not sure what you said exactly means. 
Do you want me to make a further statement for comparison between above two 
patches?
If yes, no other comments.

>> And, after MST's patch applied, the below check in 
>> virtio_pci_vq_vector_unmask() can be removed.
>> --- a/hw/virtio/virtio-pci.c    2013-05-04 15:53:20.000000000 +0800
>> +++ b/hw/virtio/virtio-pci.c    2013-05-06 10:25:58.000000000 +0800
>> @@ -619,12 +619,10 @@ static int virtio_pci_vq_vector_unmask(V
>> 
>>      if (proxy->vector_irqfd) {
>>          irqfd = &proxy->vector_irqfd[vector];
>> -        if (irqfd->msg.data != msg.data || irqfd->msg.address != 
>> msg.address) {
>>              ret = kvm_irqchip_update_msi_route(kvm_state, irqfd->virq, msg);
>>              if (ret < 0) {
>>                  return ret;
>>              }
>> -        }
>>      }
>> 
>>      /* If guest supports masking, irqfd is already setup, unmask it.
>> 
>> Thanks,
>> Zhang Haoyu