Re: [Qemu-devel] [PATCH v10 02/26] x86-iommu: provide x86_iommu_get_default

[Peter Xu <peterx@redhat.com>]

On Mon, Jul 04, 2016 at 06:16:08PM +0300, Michael S. Tsirkin wrote:
> On Tue, Jun 21, 2016 at 03:47:30PM +0800, Peter Xu wrote:
> > Instead of searching the device tree every time, one static variable is
> > declared for the default system x86 IOMMU device.  Also, some VT-d
> > macros are replaced by x86 ones.
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> 
> I think it's cleaner to just use object_resolve_path_type
> with the X86 type. Error handling by exit is rather ugly, too:
> if we need a singleton type, let's add one and have
> generic code detect such errors.

I did a quick measurement on the old path resolving method, it's
consuming >60us every time just to fetch the default IOMMU object (on
my laptop, i7-4810MQ CPU @ 2.80GHz). Do you think it'll be better if
we can avoid that? Currently there is no critical path that is using
this get_default(), only by IEC notifiers. However that will still
take some extra time during boot, or when the notifiers are triggered.

I agree that we should better provide a more general interface for
singleton semantic. But do you think it's okay I send another patch to
do that after this series merged? Since I may need some more time
reading the codes and IIUC it'll possibly be a very standalone patch
related to QOM, and another patch to just let X86 IOMMU be the first
user.

Thanks,
-- peterx