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Re: [Qemu-devel] [PATCH V3] virtio: do not require IOMMU to be created i

From: Jason Wang
Subject: Re: [Qemu-devel] [PATCH V3] virtio: do not require IOMMU to be created in advance
Date: Thu, 9 Mar 2017 10:32:44 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

On 2017年03月09日 00:40, Igor Mammedov wrote:
On Tue, 7 Mar 2017 14:47:30 +0200
Marcel Apfelbaum<address@hidden>  wrote:

On 03/07/2017 11:09 AM, Jason Wang wrote:
After commit 96a8821d2141 ("virtio: unbreak virtio-pci with IOMMU
after caching ring translations"), IOMMU was required to be created in
advance. This is because we can only get the correct dma_as after pci
IOMMU (e.g intel_iommu) was initialized. This is suboptimal and
inconvenient for user. This patch releases this by:

- introduce a bus_master_ready method for PCIDeviceClass and trigger
   this during pci_init_bus_master
- implement virtio-pci method and 1) reset the dma_as 2) re-register
   the memory listener to the new dma_as
Hi Jason,

Cc: Paolo Bonzini<address@hidden>
Signed-off-by: Jason Wang<address@hidden>
Changes from V2:
- delay pci_init_bus_master() after pci device is realized to make
   bus_master_ready a more generic method
  hw/pci/pci.c               | 11 ++++++++---
  hw/virtio/virtio-pci.c     |  9 +++++++++
  hw/virtio/virtio.c         | 19 +++++++++++++++++++
  include/hw/pci/pci.h       |  1 +
  include/hw/virtio/virtio.h |  1 +
  5 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/hw/pci/pci.c b/hw/pci/pci.c
index 273f1e4..22e6ad9 100644
--- a/hw/pci/pci.c
+++ b/hw/pci/pci.c
@@ -83,6 +83,7 @@ static const VMStateDescription vmstate_pcibus = {
  static void pci_init_bus_master(PCIDevice *pci_dev)
      AddressSpace *dma_as = pci_device_iommu_address_space(pci_dev);
+    PCIDeviceClass *pc = PCI_DEVICE_GET_CLASS(pci_dev);

                               OBJECT(pci_dev), "bus master",
@@ -90,6 +91,9 @@ static void pci_init_bus_master(PCIDevice *pci_dev)
      memory_region_set_enabled(&pci_dev->bus_master_enable_region, false);
                         &pci_dev->bus_master_enable_region, pci_dev->name);
+    if (pc->bus_master_ready) {
+        pc->bus_master_ready(pci_dev);
+    }

  static void pcibus_machine_done(Notifier *notifier, void *data)
@@ -995,9 +999,6 @@ static PCIDevice *do_pci_register_device(PCIDevice 
*pci_dev, PCIBus *bus,
      pci_dev->devfn = devfn;
      pci_dev->requester_id_cache = pci_req_id_cache_get(pci_dev);

-    if (qdev_hotplug) {
-        pci_init_bus_master(pci_dev);
-    }
      pstrcpy(pci_dev->name, sizeof(pci_dev->name), name);
      pci_dev->irq_state = 0;
@@ -1995,6 +1996,10 @@ static void pci_qdev_realize(DeviceState *qdev, Error 

+    if (qdev_hotplug) {
+        pci_init_bus_master(pci_dev);
+    }
How does it help if we move qdev_hotplug check outside "do_pci_register_device"?
I suppose you want the code to run after the "realize" function?
If so, what happens if a "realize" function of another device needs the 
(at hotplug time)?
I'm not sure that inherited device class realize
should be aware of uninitialized bus_master,
which belongs to PCI device, nor should it initialize
it by calling pci_init_bus_master() explicitly,
it's parent's class job (PCIDevice).

Yes, I was trying to propose a workaround for 2.9. I'm sure we will refine the ordering in 2.10. And consider we have asked libvirt to create IOMMU first, I think I will withdraw the patch.

more close to current code:
if xen-pci-passthrough were hotplugged then it looks like
this hunk could break it:
  memory_listener_register(&s->memory_listener, &s->dev.bus_master_as);
would happen with uninitialized bus_master_as
as realize is called before pci_init_bus_master();

Yes, this won't work. This is exactly the same issue as virtio, but this will also break if it was created before an IOMMU.

So the same question as Marcel's but other way around,
why this hunk "has to" be moved.



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