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

[Michael S. Tsirkin]

On Mon, May 06, 2013 at 01:00:09PM +0300, Michael S. Tsirkin wrote:
> On Mon, May 06, 2013 at 02:52:37AM +0000, Zhanghaoyu (A) wrote:
> > >> 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?

Ping.
Is this patch helpful to you?
If yes what is the performance gain from it?


> > 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