Re: [Xen-devel] [PATCH v2 03/11] IOMMU/MMU: enhance the call trees of IOMMU unmapping and mapping

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

On April 27, 2016 5:37 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> >>> On 27.04.16 at 10:49, <quan.xu@xxxxxxxxx> wrote:
> > On April 25, 2016 5:51 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> >> >>> On 18.04.16 at 16:00, <quan.xu@xxxxxxxxx> wrote:
> >> > --- 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 ( unlikely(ret) )
> >> > +        rc = ret;
> >>
> >> Please don't overwrite the more relevant "rc" value in situations
> >> like this
> > in
> >> case that is already non-zero. In this specific case you can actually
> >> get
> > away
> >> without introducing a second error code variable, since the only
> >> place rc
> > gets
> >> altered is between the two hunks above, and overwriting the rc value
> >> from map/unmap is then exactly what we want (but I'd much appreciate
> >> if you added a comment to this effect).
> >>
> >
> >
> > Make sense.
> > I hope I have got your point.. then, I will modify it as below:
> 
> But I screwed it up. Looking at it as you have it:
> 
> > --- a/xen/arch/x86/mm.c
> > +++ b/xen/arch/x86/mm.c
> > @@ -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)));
> > +                rc = 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);
> > +                rc = iommu_map_page(d, mfn_to_gmfn(d, page_to_mfn(page)),
> > +                                    page_to_mfn(page),
> > +                                    IOMMUF_readable|IOMMUF_writable);
> >          }
> >      }
> >
> > @@ -2593,6 +2593,8 @@ static int __get_page_type(struct page_info
> *page, unsigned long type,
> >              page->nr_validated_ptes = 0;
> >              page->partial_pte = 0;
> >          }
> > +
> > +        /* Overwrite the rc value from IOMMU map and unmap. */
> >          rc = alloc_page_type(page, type, preemptible);
> >      }
> 
> ... it is clear that this is wrong: You'd now also discard an error from the
> map/unmap with possible success coming back here. I.e. you want to
> overwrite the earlier rc only when you get a non-zero value back here.
> 

A little bit confused.
 I need to introduce a second error code variable for this point, right?
Also I shouldn't overwrite the relevant 'rc' value in case that is already 
non-zero.

I want to send out my modification for this point, making sure we are on the 
same page, even I may be still wrong.
Then this code is:

--- 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);
         }
     }

@@ -2594,6 +2594,9 @@ static int __get_page_type(struct page_info *page, 
unsigned long type,
             page->partial_pte = 0;
         }
         rc = alloc_page_type(page, type, preemptible);
+
+        if ( !rc )
+            rc = ret;
     }

     if ( (x & PGT_partial) && !(nx & PGT_partial) )




> > btw, I am clumsy at comment, I'd be pleased if you could help me enhance it.
> 
> With the above, the comment is not needed anymore.
> 
I will drop this comment.


Quan

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