Re: [Xen-devel] [v7][RFC][PATCH 08/13] xen/x86/p2m: set p2m_access_n for reserved device memory mapping

["Chen, Tiejun" <tiejun.chen@xxxxxxxxx>]

On 2014/11/4 16:02, Jan Beulich wrote:
> > > > On 04.11.14 at 02:35, <tiejun.chen@xxxxxxxxx> wrote:
> > On 2014/11/3 19:53, Jan Beulich wrote:
> > > > > > On 03.11.14 at 12:48, <tiejun.chen@xxxxxxxxx> wrote:
> > > > On 2014/11/3 18:03, Jan Beulich wrote:
> > > > > > > > On 03.11.14 at 10:51, <tiejun.chen@xxxxxxxxx> wrote:
> > > > > > On 2014/11/3 17:00, Jan Beulich wrote:
> > > > > > > > > > On 03.11.14 at 07:20, <tiejun.chen@xxxxxxxxx> wrote:
> > > > > > > > #2 the error handling
> > > > > > > >
> > > > > > > > In an error case what should I do? Currently we still create these
> > > > > > > > mapping as normal. This means these mfns will be valid so later we can't
> > > > > > > > set them again then device can't be assigned as passthrough. I think
> > > > > > > > this makes sense. Or we should just stop them from setting 1:1 mapping?
> > > > > > > You should, with very few exceptions, not ignore errors (which
> > > > > > > includes "handling" them by just logging a message. Instead, you
> > > > > > > should propagate the error back up the call chain.
> > > > > > Do you mean in your patch,
> > > > > >
> > > > > > +int iommu_get_reserved_device_memory(iommu_grdm_t *func, void *ctxt)
> > > > > > +{
> > > > > > +    const struct iommu_ops *ops = iommu_get_ops();
> > > > > > +
> > > > > > +    if ( !iommu_enabled || !ops->get_reserved_device_memory )
> > > > > > +        return 0;
> > > > > > +
> > > > > > +    return ops->get_reserved_device_memory(func, ctxt);
> > > > > > +}
> > > > > > +
> > > > > >
> > > > > > I shouldn't return that directly. Then instead, we should handle all
> > > > > > error scenarios here?
> > > > > No. All error scenarios are already being handled here (by
> > > > > propagating the error code to the caller).
> > > > Sorry, how to propagate the error code?
> > > Return it to the caller (and it will do so onwards, until it reaches
> > > [presumably] the entity having invoked a hypercall).
> > I guess you mean we should return out in this error case,
> Yes!
>
> > @@ -686,8 +686,25 @@ guest_physmap_add_entry(struct domain *d, unsigned long 
> > gfn,
> >        /* Now, actually do the two-way mapping */
> >        if ( mfn_valid(_mfn(mfn)) )
> >        {
> > -        rc = p2m_set_entry(p2m, gfn, _mfn(mfn), page_order, t,
> > -                           p2m->default_access);
> > +        rc = 0;
> > +        a =  p2m->default_access;
> > +        if ( !is_hardware_domain(d) )
> > +        {
> > +            rc = 
> > iommu_get_reserved_device_memory(p2m_check_reserved_device_memory,
> > +                                                  &gfn);
> > +            /* We always avoid populating reserved device memory. */
> > +            if ( rc == 1 )
> > +                goto out;
> But you'll need to make sure that you don't return 1 to the callers:
You're right.

> They expect 0 or negative error codes. But with the model of
> not even populating these regions (or relocating the memory
> before [at boot time] assigning a device associated with an RMRR)
> I think this needs to become an error anyway.
Looks -EINVAL might be used.

> > +            else if ( rc < 0 )
> > +            {
> > +                printk(XENLOG_G_WARNING
> > +                       "Dom%d can't check reserved device memory.\n",
> Actually, d being the subject domain, please make this more like
> "Can't check reserved device memory for Dom%d\n".
Fixed.

Thanks
Tiejun

> Jan
>
> > +                       d->domain_id);
> > +                goto out;
> > +            }
> > +        }
> > +
> > +        rc = p2m_set_entry(p2m, gfn, _mfn(mfn), page_order, t, a);
> >            if ( rc )
> >                goto out; /* Failed to update p2m, bail without updating m2p. */
> >
> > Thanks
> > Tiejun
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