Re: [Xen-devel] [PATCH v4 02/10] IOMMU: handle IOMMU mapping and unmapping failures

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

On May 10, 2016 2:54 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> >>> On 10.05.16 at 05:41, <quan.xu@xxxxxxxxx> wrote:
> > On May 10, 2016 12:14 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> >> >>> On 06.05.16 at 10:54, <quan.xu@xxxxxxxxx> wrote:
> >> > --- a/xen/drivers/passthrough/iommu.c
> >> > +++ b/xen/drivers/passthrough/iommu.c
> >> > @@ -240,21 +240,47 @@ int iommu_map_page(struct domain *d,
> >> unsigned long gfn, unsigned long mfn,
> >> >                     unsigned int flags)  {
> >> >      const struct domain_iommu *hd = dom_iommu(d);
> >> > +    int rc;
> >> >
> >> >      if ( !iommu_enabled || !hd->platform_ops )
> >> >          return 0;
> >> >
> >> > -    return hd->platform_ops->map_page(d, gfn, mfn, flags);
> >> > +    rc = hd->platform_ops->map_page(d, gfn, mfn, flags);
> >> > +
> >> > +    if ( unlikely(rc) )
> >> > +    {
> >> > +        printk(XENLOG_ERR
> >> > +               "iommu_map_page: IOMMU mapping gfn %#lx mfn %#lx
> >> > + failed for
> >> dom%d.",
> >> > +               gfn, mfn, d->domain_id);
> >> > +
> >> > +        if ( !is_hardware_domain(d) )
> >> > +            domain_crash(d);
> >> > +    }
> >>
> >> This still may spam the console in at least the case of Dom0.
> >
> > I am afraid we may need a minor trade-off. What about:
> >
> >        dprintk(XENLOG_ERR, "...");
> >
> > to print out in debug mode.
> 
> And be silent in non-debug mode? That's not what we want.
> 

Without your below suggestion, this is really my best solution.

> >>  For DomU I'd
> >> really expect you to state in the commit message why no spamming can
> >> occur (of course assuming it really can't, which I'm not convinced of).
> >>
> >
> > In this v4, I think we will still spam the console in extreme cases :(:(..
> >
> > For mapping:
> > +                ret = iommu_map_page();
> > +                if ( unlikely(ret) )
> > +                {
> > +                    while ( i-- )
> > +                        iommu_unmap_page();
> > +                }
> >
> > We'll  stop map against any error and unmapping the previous mappings.
> > The extreme case is error for unmapping the previous mappings.
> >
> > Again -- I think dprintk is a better solution. Any suggestion?
> 
> For DomU the solution seems quite obvious: Only log a message if the domain
> is not already marked crashed. For Dom0 you'll need to get a little more
> creative (but by leveraging the fact that there's only one in the system, this
> can't be too difficult a problem to solve:
> e.g. "manually" rate limit these messages - see printk_ratelimit() et al).

Amazing!!
As the comment said, printk_ratelimit() is lifted from Linux. referred to the 
Linux, __iiuc__ , I will fix this issue as below (a variant):

...
+    rc = hd->platform_ops->unmap_page(d, gfn);
+
+    if ( unlikely(rc) )
+    {
+        if ( printk_ratelimit() )
+            printk(XENLOG_ERR
+                   "iommu_unmap_page: IOMMU unmapping gfn %#lx failed for 
dom%d.",
+                   gfn, d->domain_id);
+
+        if ( !is_hardware_domain(d) )
+            domain_crash(d);
+    }
+
+    return rc;
...

Thanks!!



Quan

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