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Re: [libvirt] [PATCH 2/2] qemu: Don't fail if the SCSI host device is shareable between domains



On 06/01/14 23:54, John Ferlan wrote:

On 01/02/2014 09:45 AM, Osier Yang wrote:
It doesn't make sense to fail if the SCSI host device is specified
as "shareable" explicitly between domains (NB, it works if and only
if the device is specified as "shareable" for *all* domains,
otherwise it fails).

Also don't try to add the device to the activeScsiHostdevs list if
it's already there.
---
  src/qemu/qemu_hostdev.c | 33 ++++++++++++++++++---------------
  1 file changed, 18 insertions(+), 15 deletions(-)

diff --git a/src/qemu/qemu_hostdev.c b/src/qemu/qemu_hostdev.c
index 86a463a..8536499 100644
--- a/src/qemu/qemu_hostdev.c
+++ b/src/qemu/qemu_hostdev.c
@@ -1120,22 +1120,25 @@ qemuPrepareHostdevSCSIDevices(virQEMUDriverPtr driver,
          if ((tmp = virSCSIDeviceListFind(driver->activeScsiHostdevs, scsi))) {
              const char *other_name = virSCSIDeviceGetUsedBy(tmp);
- if (other_name)
-                virReportError(VIR_ERR_OPERATION_INVALID,
-                               _("SCSI device %s is in use by domain %s"),
-                               virSCSIDeviceGetName(tmp), other_name);
-            else
-                virReportError(VIR_ERR_OPERATION_INVALID,
-                               _("SCSI device %s is already in use"),
-                               virSCSIDeviceGetName(tmp));
-            goto error;
-        }
-
-        virSCSIDeviceSetUsedBy(scsi, name);
-        VIR_DEBUG("Adding %s to activeScsiHostdevs", virSCSIDeviceGetName(scsi));
+            if (!(virSCSIDeviceGetShareable(scsi) &&
+                  virSCSIDeviceGetShareable(tmp))) {
+                if (other_name)
+                    virReportError(VIR_ERR_OPERATION_INVALID,
+                                   _("SCSI device %s is in use by domain %s"),
+                                   virSCSIDeviceGetName(tmp), other_name);
+                else
+                    virReportError(VIR_ERR_OPERATION_INVALID,
+                                   _("SCSI device %s is already in use"),
+                                   virSCSIDeviceGetName(tmp));
+                goto error;
+            }
+        } else {
+            virSCSIDeviceSetUsedBy(scsi, name);
+            VIR_DEBUG("Adding %s to activeScsiHostdevs", virSCSIDeviceGetName(scsi));
- if (virSCSIDeviceListAdd(driver->activeScsiHostdevs, scsi) < 0)
-            goto error;
+            if (virSCSIDeviceListAdd(driver->activeScsiHostdevs, scsi) < 0)
+                goto error;
+        }

Something doesn't feel right here...

Prior code:

if find device on some other drivers active list
     fail
set name of who device is used by
add device to host active list
   (NOTE: will error if found on list already)


New code

if find device on some other drivers active list
     if both devices not marked shareable
         fail
else
     set name of who device is used by
     add device to host active list


So now theoretically with a shareable device, the device could be on the
active list and it's OK to use it because it's marked shareable.

Yes, as long as the later domain(s) specify the device as shareable
too.


So my question/issue becomes what to do with the "dev->used_by" (eg,
virSCSIDeviceSetUsedBy() result) value.  In fact, since we're now
shareable it seems to me that we'd need to keep this up to date even
going so far as to clear usage it when virSCSIDeviceListDel() is called.
Should the list become a "list" of those using it?

/Alarm....

Yes, thanks for pointing it out...

The first domain
that claims usage could eventually remove their usage, then what? It
would be strange to report domain X has the device in use if the domain
is down or doesn't exist.

It actually won't report error. Since the "dev->used_by" is freed
along with the domain destroying. E.g.

% start domain A with sg7 as shareable.

% start domain B with sg7 as shareable

% Destroy domain A

% start domain C with sg7 as non-shareable.

Starting Domain C is expected to fail, but it will succeed.

I will post the v2.

  Use cscope and check the callers/users of
"used_by" to see what my concern is.

Remember prior to this point in time we cannot share, so it didn't
matter. However, from this point on since we can share we need to keep
track of who has it, right?  What becomes even more interesting perhaps
is the impact on migration (if that's even possible with a shared hostdev).

It should be fine for migration, as long as we make the "used_by"
correct.

Regards,
Osier



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