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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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