On 31/07/2021 16:41, Jason Thorpe wrote:
(added Michael on CC)
Hey folks —
I’d like to be able to use VirtIO with qemu-system-alpha but, at least on a NetBSD
x86_64 host, it does not currently work. This is because virtio_bus_device_plugged()
in hw/virtio/virtio-bus.c ends up picking address_space_memory as the DMA address
space for the VirtIODevice. This does not work for alpha because the CPU and PCI
have different views of system memory. All that’s needed to fix it is for
virtio_bus_device_plugged() to call klass->get_dma_as(qbus->parent), but the
code only does that if:
bool has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
So, obviously, VIRTIO_F_IOMMU_PLATFORM is not getting set for an emulated alpha
system, despite the alpha platform having one[*]. But it’s not clear to me
that it’s appropriate for alpha to use VIRTIO_F_IOMMU_PLATFORM, at least from
my reading of how it’s used.
In any case, the following extremely simple change allows me to use VirtIO
devices in qemu-system-alpha with a NetBSD/alpha guest (and I’m told this also
fixes using VirtIO devices in qemu-system-sparc64 for a NetBSD/sparc64 guest):
diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
index 859978d248..c083e8d737 100644
--- a/hw/virtio/virtio-bus.c
+++ b/hw/virtio/virtio-bus.c
@@ -85,6 +85,8 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error
**errp)
if (klass->get_dma_as != NULL && has_iommu) {
virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
vdev->dma_as = klass->get_dma_as(qbus->parent);
+ } else if (klass->get_dma_as != NULL) {
+ vdev->dma_as = klass->get_dma_as(qbus->parent);
} else {
vdev->dma_as = &address_space_memory;
}
So, VirtIO experts, please weigh in on the correctness of this change… if it
is, I’ll post the patch formally.
[*] The way the alpha platform works is that the IOMMU is used if the PCI
device performs a memory access to a DMA window where SGMAPs are enabled. If
SGMAPs are not enabled in the DMA window the PCI device is accessing, the
translation is performed directly by subtracting the address from the window’s
Window Base and appending the result to the window’s Translated Base. A
typical alpha PCI platform has a 1GB DMA window at 1GB from the PCI’s
perspective, which maps to 0-1GB in the system address map, and an alpha system
with 1GB or less of RAM would thus not need to use the IOMMU, but the
translation take place regardless.
-- thorpej
Hi Jason,
Thanks for looking at this! I've had previous discussions with Martin trying to
figure out why virtio-blk-pci doesn't work with Netbsd/sparc64 so glad that you've
been able to find the underlying cause.
My read on VIRTIO_F_IOMMU_PLATFORM is that this is declaring host IOMMU support so
the implementation on the guest should not be relevant here.
Testing Linux/sparc64 boot from a virtio-blk-pci device on current git master shows I
can boot from a virtio-blk-pci device without any problem, even though the existing
code fails the has_iommu check and vdev->dma_as gets set to address_space_memory
which is why I haven't spotted this before.
Stepping through the code shows that klass->get_dma_as() is pointing to
virtio_pci_get_dma_as() which in turn returns pci_get_address_space(dev) which looks
correct to me. I think that the logic to set vdev->dma_as is incorrect here since
qemu-system-sparc64 has an emulated IOMMU with its own address space independent of
whether the host has an IOMMU.
I modified your patch slightly as below and confirmed that I can still boot
Linux/sparc64 here:
diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
index 859978d248..ee178b8223 100644
--- a/hw/virtio/virtio-bus.c
+++ b/hw/virtio/virtio-bus.c
@@ -82,9 +82,11 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error
**errp)
return;
}
- if (klass->get_dma_as != NULL && has_iommu) {
- virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
+ if (klass->get_dma_as != NULL) {
vdev->dma_as = klass->get_dma_as(qbus->parent);
+ if (has_iommu) {
+ virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
+ }
} else {
vdev->dma_as = &address_space_memory;
}
Michael: can you comment further on whether the analysis and patch above are
correct?
ATB,
Mark.
