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Re: [Qemu-devel] [PATCH v3] intel_iommu: check misordered init when real

From: Marcel Apfelbaum
Subject: Re: [Qemu-devel] [PATCH v3] intel_iommu: check misordered init when realize
Date: Thu, 2 Mar 2017 08:24:44 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1

On 03/02/2017 07:13 AM, Michael S. Tsirkin wrote:
On Thu, Mar 02, 2017 at 11:32:18AM +0800, Peter Xu wrote:
Intel vIOMMU devices are created with "-device" parameter, while here
actually we need to make sure the dmar device be created before other
PCI devices (like vfio-pci, virtio-pci ones) so that we know iommu_fn
will be setup correctly before realizations of those PCI devices (it is
legal that PCI device fetch these info during its realization). Now this
ordering yet cannot be achieved elsewhere, and devices will be created
in the order that user specified. We need to avoid that.

This patch tries to detect this kind of misordering issue during init of
VT-d device, then report to guest if misordering happened. In the
future, we can provide something better to solve it, e.g., to support
device init ordering, then we can live without this patch.

Signed-off-by: Peter Xu <address@hidden>

Unfortunately with virtio it's a regression, as it used to
work with iommu. So I'm afraid we need to look into supporting
arbitrary order right now :(


A fast way to do it is to initialize iommu with a new keyword, like
    -iommu intel-iommu
Or maybe use the existing "-object" somehow ?
From vl.c comments:
  "Initial object creation happens before all other
   QEMU data types are created.... "

Another idea is to add a third run on QEMU cmd line arguments,
but this would affect the whole system.

However having an ordering system beats all other ideas.


v3: added virtio-pci device detection since we have that requirement as
    well now.

PS. this patch should be needed along with Jason's:
 "virtio: unbreak virtio-pci with IOMMU after caching ring translations"
to make sure virtio devices with vt-d are safe for 2.9.
 hw/i386/intel_iommu.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
index 22d8226..1077f90 100644
--- a/hw/i386/intel_iommu.c
+++ b/hw/i386/intel_iommu.c
@@ -2560,6 +2560,24 @@ static bool vtd_decide_config(IntelIOMMUState *s, Error 
     return true;

+ * TODO: we should have a better way to achieve the ordering rather
+ * than this misorder check explicitly against vfio-pci and virtio-pci
+ * devices. Here, there's no easy way to detect init of virtio-pci
+ * devices, instead we detect the virtio bus.
+ */
+static bool vtd_detected_misorder_init(Error **errp)
+    if (object_resolve_path_type("", "vfio-pci", NULL) ||
+        object_resolve_path_type("", "virtio-pci-bus", NULL)) {
+        error_setg(errp, "Please specify \"intel-iommu\" before "
+                   "all the rest of the devices.");
+        return true;
+    }
+    return false;
 static void vtd_realize(DeviceState *dev, Error **errp)
     PCMachineState *pcms = PC_MACHINE(qdev_get_machine());

We'd put this somewhere central - this hack is not vtd specific.

@@ -2567,6 +2585,10 @@ static void vtd_realize(DeviceState *dev, Error **errp)
     IntelIOMMUState *s = INTEL_IOMMU_DEVICE(dev);
     X86IOMMUState *x86_iommu = X86_IOMMU_DEVICE(dev);

+    if (vtd_detected_misorder_init(errp)) {
+        return;
+    }
     x86_iommu->type = TYPE_INTEL;


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