Re: [Qemu-devel] [PATCH qemu v5 04/12] spapr_pci_vfio: Enable multiple groups per container

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Apr 08, 2015 at 01:45:19PM +1000, Alexey Kardashevskiy wrote:
> On 04/08/2015 12:01 PM, David Gibson wrote:
> >On Tue, Mar 31, 2015 at 04:28:39PM +1100, Alexey Kardashevskiy wrote:
> >>This enables multiple IOMMU groups in one VFIO container which means
> >>that multiple devices from different groups can share the same IOMMU
> >>table (or tables if DDW).
> >>
> >>This removes a group id from vfio_container_ioctl(). The kernel support
> >>is required for this; if the host kernel does not have the support,
> >>it will allow only one group per container. The PHB's "iommuid" property
> >>is ignored.
> >>
> >>This adds a sanity check that there is just one VFIO container per
> >>PHB address space.
> >>
> >>Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> >
> >[snip]
> >>diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> >>index b012620..99e1900 100644
> >>--- a/hw/vfio/common.c
> >>+++ b/hw/vfio/common.c
> >>@@ -915,21 +915,23 @@ void vfio_put_base_device(VFIODevice *vbasedev)
> >>      close(vbasedev->fd);
> >>  }
> >>
> >>-static int vfio_container_do_ioctl(AddressSpace *as, int32_t groupid,
> >>+static int vfio_container_do_ioctl(AddressSpace *as,
> >>                                     int req, void *param)
> >>  {
> >>-    VFIOGroup *group;
> >>      VFIOContainer *container;
> >>-    int ret = -1;
> >>+    int ret;
> >>+    VFIOAddressSpace *space;
> >>
> >>-    group = vfio_get_group(groupid, as);
> >>-    if (!group) {
> >>-        error_report("vfio: group %d not registered", groupid);
> >>-        return ret;
> >>-    }
> >>+    space = vfio_get_address_space(as);
> >>+    container = QLIST_FIRST(&space->containers);
> >
> >So getting the container handle from the address space, rather than
> >the group id certainly makes more sense to me.
> >
> >>-    container = group->container;
> >>-    if (group->container) {
> >>+    if (!container) {
> >>+        error_report("vfio: container is not set");
> >>+        return -1;
> >>+    } else if (QLIST_NEXT(container, next)) {
> >>+        error_report("vfio: multiple containers per PHB are not supported");
> >>+        return -1;
> >
> >But if only one PHB per address space is possible, why is the
> >containers field a list in the first place?
> 
> 
> Historically the list was added in 3df3e0a5872 (the patch of yours
> :) ).

Heh.

> In theory we could implement spapr-pci-bridge (derived from pci-bridge) with
> isolation capability (i.e. its own LIOBN/DMA window), in this case there
> could be multiple containers per PHB address space. Other archs could want
> multiple containers for some other reason. It would help me a lot if you
> remembered why you kept the list at the first place :)

Ok, I've looked over the patch and it has jogged my memory a bit.  So
the dumb answer is that it's because the per address-space list was
replacing a global list of containers

The more useful answer is that I think it was because I was
anticipating the possibility of working around the
one-group-per-container limit by allowing a single VFIOAddressSpace in
qemu to be backed by several containers, whose mappings would be kept
in sync from the userspace side by duplicating all mappings.

Anyway, I think that means the right way to implement this is by
duplicating the ioctl() across all the attached containers, rather
than picking just one.

> For now I guess I'll move the next patch ("vfio: spapr: Move SPAPR-related
> code to a separate file") before this one, do s/vfio_container_do_ioctl/
> vfio_spapr_container_do_ioctl/ and move it to hw/vfio/spapr.c. Makes
> sense?

That sounds fine, though I don't see that it really addresses the
question here.


> 
> 
> >>+    } else {
> >>          ret = ioctl(container->fd, req, param);
> >>          if (ret < 0) {
> >>              error_report("vfio: failed to ioctl %d to container: ret=%d, %s",
> >>@@ -937,12 +939,10 @@ static int vfio_container_do_ioctl(AddressSpace *as, int32_t groupid,
> >>          }
> >>      }
> >>
> >>-    vfio_put_group(group);
> >>-
> >>      return ret;
> >>  }
> >>
> >>-int vfio_container_ioctl(AddressSpace *as, int32_t groupid,
> >>+int vfio_container_ioctl(AddressSpace *as,
> >>                           int req, void *param)
> >>  {
> >>      /* We allow only certain ioctls to the container */
> >>@@ -957,5 +957,5 @@ int vfio_container_ioctl(AddressSpace *as, int32_t groupid,
> >>          return -1;
> >>      }
> >>
> >>-    return vfio_container_do_ioctl(as, groupid, req, param);
> >>+    return vfio_container_do_ioctl(as, req, param);
> >>  }
> >>diff --git a/include/hw/vfio/vfio.h b/include/hw/vfio/vfio.h
> >>index 0b26cd8..76b5744 100644
> >>--- a/include/hw/vfio/vfio.h
> >>+++ b/include/hw/vfio/vfio.h
> >>@@ -3,7 +3,7 @@
> >>
> >>  #include "qemu/typedefs.h"
> >>
> >>-extern int vfio_container_ioctl(AddressSpace *as, int32_t groupid,
> >>+extern int vfio_container_ioctl(AddressSpace *as,
> >>                                  int req, void *param);
> >>
> >>  #endif
> >
> 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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