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Re: [libvirt] [PATCH 1/2] qemu: Add capability flag for usb-storage



13/08/13 16:15, Daniel P. Berrange wrote:
> On Tue, Aug 13, 2013 at 01:52:33PM +0200, Fred A. Kemp wrote:
>> From: "Fred A. Kemp" <anonym riseup net>
>>
>> Allow use of the usb-storage device only if the new capability flag
>> QEMU_CAPS_DEVICE_USB_STORAGE is set, which it is for qemu(-kvm)
>> versions >= 0.12.1.2-rhel62-beta.
>> ---
>>  src/qemu/qemu_capabilities.c |    2 ++
>>  src/qemu/qemu_capabilities.h |    1 +
>>  src/qemu/qemu_command.c      |    6 +++---
>>  tests/qemuhelptest.c         |   18 ++++++++++++------
>>  tests/qemuxml2argvtest.c     |    3 ++-
>>  5 files changed, 20 insertions(+), 10 deletions(-)
>>
>> diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
>> index 47cc07a..5c566c1 100644
>> --- a/src/qemu/qemu_capabilities.c
>> +++ b/src/qemu/qemu_capabilities.c
>> @@ -235,6 +235,7 @@ VIR_ENUM_IMPL(virQEMUCaps, QEMU_CAPS_LAST,
>>                "vnc-share-policy", /* 150 */
>>                "device-del-event",
>>                "dmi-to-pci-bridge",
>> +              "usb-storage",
>>      );
>>  
>>  struct _virQEMUCaps {
>> @@ -1383,6 +1384,7 @@ struct virQEMUCapsStringFlags virQEMUCapsObjectTypes[] = {
>>      { "vfio-pci", QEMU_CAPS_DEVICE_VFIO_PCI },
>>      { "scsi-generic", QEMU_CAPS_DEVICE_SCSI_GENERIC },
>>      { "i82801b11-bridge", QEMU_CAPS_DEVICE_DMI_TO_PCI_BRIDGE },
>> +    { "usb-storage", QEMU_CAPS_DEVICE_USB_STORAGE },
>>  };
>>  
>>  static struct virQEMUCapsStringFlags virQEMUCapsObjectPropsVirtioBlk[] = {
>> diff --git a/src/qemu/qemu_capabilities.h b/src/qemu/qemu_capabilities.h
>> index 074e55d..4a8b14b 100644
>> --- a/src/qemu/qemu_capabilities.h
>> +++ b/src/qemu/qemu_capabilities.h
>> @@ -191,6 +191,7 @@ enum virQEMUCapsFlags {
>>      QEMU_CAPS_VNC_SHARE_POLICY   = 150, /* set display sharing policy */
>>      QEMU_CAPS_DEVICE_DEL_EVENT   = 151, /* DEVICE_DELETED event */
>>      QEMU_CAPS_DEVICE_DMI_TO_PCI_BRIDGE  = 152, /* -device i82801b11-bridge */
>> +    QEMU_CAPS_DEVICE_USB_STORAGE = 153, /* -device usb-storage */
>>  
>>      QEMU_CAPS_LAST,                   /* this must always be the last item */
>>  };
>> diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
>> index b811e1d..03fb798 100644
>> --- a/src/qemu/qemu_command.c
>> +++ b/src/qemu/qemu_command.c
>> @@ -8044,10 +8044,10 @@ qemuBuildCommandLine(virConnectPtr conn,
>>              bool withDeviceArg = false;
>>              bool deviceFlagMasked = false;
>>  
>> -            /* Unless we have -device, then USB disks need special
>> -               handling */
>> +            /* Unless we have `-device usb-storage`, then USB disks
>> +               need special handling */
>>              if ((disk->bus == VIR_DOMAIN_DISK_BUS_USB) &&
>> -                !virQEMUCapsGet(qemuCaps, QEMU_CAPS_DEVICE)) {
>> +                !virQEMUCapsGet(qemuCaps, QEMU_CAPS_DEVICE_USB_STORAGE)) {
>>                  if (disk->device == VIR_DOMAIN_DISK_DEVICE_DISK) {
>>                      virCommandAddArg(cmd, "-usbdevice");
>>                      virCommandAddArgFormat(cmd, "disk:%s", disk->src);
> 
> Hmm, I'm not sure this logic change is right.
> 
> Previously if you have -device support, but 'usb-storage' was not
> available, libvirt would try to start the guest with -device & then
> QEMU would report an error.
> 
> With this change, if you have -device support, but 'usb-storage' was
> not available, then libvirt is going to fallback to using the legacy
> '-usbdevice' support. This is not good.

I fail to see why this is not good. I thought '-usbdevice' was the
correct way do handle USB disks if 'usb-storage' is missing. Whether we
have '-device' or seems beside the point; if we have 'usb-storage', then
it's implied we have '-device', and if we don't, '-device' is worthless
and '-usbdevice' is the way to go. Letting QEMU fail and die when we
have '-device' but not 'usb-storage' seems worse than just falling back
to '-usbdevice'.

What am I missing?

> Adding an explicit check for 'usb-storage' is a fine goal, but we
> should be doing that check in the branch of this  if() that handles
> '-device usb-storage', so we don't exercise the -usbdevice branch

This doesn't make sense to me either, but I guess it will after clearing
up my previous confusion.

Cheers!


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