Re: [Xen-devel] [PATCH v3 03/10] IOMMU/MMU: enhance the call trees of IOMMU unmapping and mapping

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

> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Wednesday, May 04, 2016 5:29 PM
> 
> >>> On 04.05.16 at 11:13, <quan.xu@xxxxxxxxx> wrote:
> > On May 04, 2016 4:41 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> >> >>> On 04.05.16 at 03:45, <kevin.tian@xxxxxxxxx> wrote:
> >> >>  From: Xu, Quan
> >> >> Sent: Friday, April 29, 2016 5:25 PM
> >> >> --- 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;
> >> >> +
> >> >>      return rc;
> >> >>  }
> >> >
> >> > I know there were quite some discussions before around above change
> >> > (sorry I didn't remember all of them). Just based on mental picture we
> >> > should return error where it firstly occurs. However above change
> >> > looks favoring errors in later "rc = alloc_page_type" over earlier
> >> > iommu_map/unmap_page error. Is it what we want?
> >>
> >> Yes, as that's the primary operation here.
> >>
> >> > If there is a reason that we cannot return immediately upon
> >> > iommu_map/unmap,
> >>
> >> Since for Dom0 we don't call domain_crash(), we must not bypass
> >> alloc_page_type() here. And even for DomU it would seem at least fragile 
> >> if we
> >> did - we better don't alter the refcounting behavior.
> >>
> >
> > A little bit confused.
> > Check it,  for this __get_page_type(), can I leave my modification as is?
> 
> Yes. I merely tried to explain the reason to Kevin.
> 

Understand now.

Thanks
Kevin

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