Re: [Xen-devel] [PATCH v9 2/3] VT-d: wrap a _sync version for all VT-d flush interfaces

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Xu, Quan
> Sent: Thursday, April 07, 2016 3:45 PM
> 
> On April 05, 2016 5:35pm, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> > >>> On 01.04.16 at 16:47, <quan.xu@xxxxxxxxx> wrote:
> > > The dev_invalidate_iotlb() scans ats_devices list to flush ATS
> > > devices, and the invalidate_sync() is put after dev_invalidate_iotlb()
> > > to synchronize with hardware for flush status. If we assign multiple
> > > ATS devices to a domain, the flush status is about all these multiple
> > > ATS devices. Once flush timeout expires, we couldn't find out which
> > > one is the buggy ATS device.
> >
> > Is that true? Or is that just a limitation of our implementation?
> >
> 
> IMO, both.
> I hope vt-d maintainers help me double check it.

Just a limitation of our implementation. Now dev_invalidate_iotlb puts
all possible IOTLB flush requests to the queue, and then invalidate_sync
pushes a wait descriptor w/ timeout to detect error. VT-d spec says one
or more descriptors may be fetched together by hardware. So when a 
timeout is triggered, we cannot tell which flush request actually has 
problem by reading queue head register. If we change the implementation
to one-invalidation-sync-per-request, then we can tell. I discussed with
Quan not to go that complexity though.

Thanks
Kevin
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