Re: [Xen-devel] [PATCH 1/2] IOMMU/MMU: Adjust top level functions for VT-d Device-TLB flush error.

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

On March 18, 2016 4:20pm, <JBeulich@xxxxxxxx> wrote:
> >>> On 18.03.16 at 08:54, <quan.xu@xxxxxxxxx> wrote:
> > On March 17, 2016 8:30pm, <dunlapg@xxxxxxxxx> wrote:
> >> On Thu, Mar 17, 2016 at 6:54 AM, Quan Xu <quan.xu@xxxxxxxxx> wrote:
> >> > diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c index
> >> > c997b53..526548e 100644
> >> > --- a/xen/arch/x86/mm.c
> >> > +++ b/xen/arch/x86/mm.c
> >> > @@ -2467,7 +2467,7 @@ static int __get_page_type(struct page_info
> >> > *page,
> >> unsigned long type,
> >> >                             int preemptible)  {
> >> >      unsigned long nx, x, y = page->u.inuse.type_info;
> >> > -    int rc = 0;
> >> > +    int rc = 0, ret = 0;
> >> >
> >> >      ASSERT(!(type & ~(PGT_type_mask | PGT_pae_xen_l2)));
> >> >
> >> > @@ -2578,11 +2578,11 @@ static int __get_page_type(struct page_info
> >> *page, unsigned long type,
> >> >          if ( d && is_pv_domain(d) && unlikely(need_iommu(d)) )
> >> >          {
> >> >              if ( (x & PGT_type_mask) == PGT_writable_page )
> >> > -                iommu_unmap_page(d, mfn_to_gmfn(d,
> >> page_to_mfn(page)));
> >> > +                ret = iommu_unmap_page(d, mfn_to_gmfn(d,
> >> page_to_mfn(page)));
> >> >              else if ( type == PGT_writable_page )
> >> > -                iommu_map_page(d, mfn_to_gmfn(d,
> >> page_to_mfn(page)),
> >> > -                               page_to_mfn(page),
> >> > -
> >> IOMMUF_readable|IOMMUF_writable);
> >> > +                ret = iommu_map_page(d, mfn_to_gmfn(d,
> >> page_to_mfn(page)),
> >> > +                                     page_to_mfn(page),
> >> > +
> >> IOMMUF_readable|IOMMUF_writable);
> >> >          }
> >> >      }
> >> >
> >> > @@ -2599,6 +2599,9 @@ static int __get_page_type(struct page_info
> >> > *page,
> >> unsigned long type,
> >> >      if ( (x & PGT_partial) && !(nx & PGT_partial) )
> >> >          put_page(page);
> >> >
> >> > +    if ( !rc )
> >> > +        rc = ret;
> >> > +
> >>
> >> What's this about?  If the iommu_[un]map_page() operation times out,
> >> we still go through with calling alloc_page_type(); and if
> >> alloc_page_type() fails we return its failure value, but if it
> >> succeeds, we return the error from iommu_[un]map_page()?
> >>
> > Yes.
> > To be honest, to me, it is tricky too.
> > I found it is not right to return directly if the iommu_[un]map_page()
> > operation times out.
> >
> >  """if ( d && is_pv_domain(d) && unlikely(need_iommu(d)) )"""
> > Does IOMMU support pv domain? If not, we'd better remove the " if(...){...}"
> 
> What strange a question: Of course it does.
> 

Jan, thanks. 
To be honest, this stupid question was always in my mind.

> As you can infer form the reply I sent yesterday, you first need to settle on 
> an
> abstract model: What do failures mean? If, in the flush case, a timeout is 
> going
> to kill the domain anyway, then error handling can be lighter weight than if 
> you
> mean to try to keep the domain running. Of course in this context you also
> should not forget that iommu_map_page() could already return errors prior to
> your changes (most notably -ENOMEM, but at least the AMD side also produces
> others, with -EFAULT generally being accompanied by domain_crash()). As
> mentioned elsewhere - it seems extremely bogus that these errors weren't
> check for from the beginning.
> 
Jan, I am not familiar/confident enough to this single point, could you tell me 
more how to fix it?
Quan

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