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

["Xu, Quan" <quan.xu@xxxxxxxxx>]

On April 11, 2016 3:25pm, Tian, Kevin <kevin.tian@xxxxxxxxx> wrote:
> > From: Xu, Quan
> > Sent: Friday, April 08, 2016 10:21 AM
> >
> > On April 07, 2016 11:29pm, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> > > >>> On 07.04.16 at 09:44, <quan.xu@xxxxxxxxx> wrote:
> > > > On April 05, 2016 5:35pm, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> > > >> >>> On 01.04.16 at 16:47, <quan.xu@xxxxxxxxx> wrote:
> > > >> > +{
> > > >> > +    queue_invalidate_context(iommu, did, source_id,
> > > >> > +                             function_mask, granu);
> > > >> > +
> > > >> > +    return invalidate_sync(iommu); }
> > > >>
> > > >> Further down you replace the only call to
> > > >> queue_invalidate_context() - why keep both functions instead of
> > > >> just making
> > > > the
> > > >> existing one do the sync? (That would the likely also apply to
> > > >> qinval_device_iotlb() and others below.)
> > > >>
> > > >
> > > > It is optional.
> > > >  I think:
> > > > 1. in the long term, we may need no _sync version.
> > > > 2. At least, the current wrap looks good to me. e.g.
> > > > queue_invalidate_context() is for context-cache Invalidate
> > > > Descriptor, and the
> > > > invalidate_sync() is for Invalidation Wait Descriptor. It is much 
> > > > clearer.
> > >
> > > I don't really agree, but will leave it to the VT-d maintainers to judge.
> > >
> >
> > +to Kevin and Feng, I am open for it.
> 
> Let's just change existing one to _sync.
> 

Agreed.


> >
> >
> > > >> > +        if ( ret )
> > > >> > +            return ret;
> > > >> > +
> > > >> >          if ( flush_dev_iotlb )
> > > >> >              ret = dev_invalidate_iotlb(iommu, did, addr,
> > > >> > size_order,
> > > type);
> > > >> > -        rc = invalidate_sync(iommu);
> > > >> > -        if ( !ret )
> > > >> > -            ret = rc;
> > > >> >      }
> > > >>
> > > >> I think leaving the existing logic as is would be better - best
> > > >> effort
> > > > invalidation
> > > >> even when an error has occurred.
> > > >>
> > > >
> > > > I have an open:
> > > > As vt-d spec(:Queued Invalidation Ordering Considerations) said,
> > > >      1. If the Fence(FN) flag is 1 in a inv_wait_dsc, hardware
> > > > must execute descriptors following the inv_wait_dsc only after
> > > > wait command is
> > > completed.
> > > >      2. when a Device-TLB invalidation timeout is detected,
> > > > hardware must not complete any pending inv_wait_dsc commands.
> > > > In current code, the Fence(FN) is always 1.
> > > > if a Device-TLB invalidation timeout is detected, this additional
> > > > inv_wait_dsc is not completed.
> > > > __iiuc__,
> > > > the new coming descriptors, in that queue, _might_ be not executed
> > > > any more, waiting for this additional inv_wait_dsc which is not 
> > > > completed.
> > > > is it true?
> > >
> > > That's not a question to me, is it?
> >
> > To community, but vt-d maintainers are someone who can explain to me.
> >
> 
> 'not completed' here means 'abort', so your timeout check will be hit since 
> the
> status is never 'done' then.
> 
> But I'm not sure how your question is related to Jan's comment, which looks
> reasonable to me. :-)
> 

I will take Jan's suggestion.
For this open, at least, to remind myself, I should consider software behavior 
and hardware behavior.
Let's ignore my open.

Quan


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