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

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

On 2014/11/3 17:00, Jan Beulich wrote:
>>>> On 03.11.14 at 07:20, <tiejun.chen@intel.com> wrote:
>> On 2014/10/31 16:25, Jan Beulich wrote:
>>>>>> On 31.10.14 at 03:50, <tiejun.chen@intel.com> wrote:
>>>> On 2014/10/30 17:24, Jan Beulich wrote:
>>>>>>>> On 30.10.14 at 08:39, <tiejun.chen@intel.com> wrote:
>>>>>> @@ -686,8 +686,22 @@ 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 need to set reserved device memory as p2m_access_n. */
>>>>>> +            if ( rc == 1 )
>>>>>> +                a = p2m_access_n;
>>>>>> +            else if ( rc < 0 )
>>>>>> +                printk(XENLOG_WARNING
>>>>>> +                       "Domain %d can't check reserved device memory.\n",
>>>>>> +                       d->domain_id);
>>>>>> +        }
>>>>>> +
>>>>>> +        rc = p2m_set_entry(p2m, gfn, _mfn(mfn), page_order, t, a);
>>>>>>              if ( rc )
>>>>>>                  goto out; /* Failed to update p2m, bail without updating m2p.
>>>> */
>>>>>
>>>>> The handling of "a" looks good now, but the error handling and
>>>>> logging is still as broken as it was before.
>>>>
>>>> Do you mean I'm missing some necessary info? Like gfn and mfn, so domain
>>>> id, gfn and mfn can show enough message.
>>>>
>>>> Sorry I'm poor to understand what you expect.
>>>
>>> But I explained it already, and that explanation is still visible in
>>> the quotes above. But to avoid any doubt, I'll repeat: "And
>>
>> I tried to understand what you said but felt a confusion so ask if you
>> show me directly.
>>
>>> properly handle the error case (just logging a message - which
>>> btw lacks a proper XENLOG_G_* prefix - doesn't seem enough
>>> to me)."
>>
>> Looks there are two problems:
>>
>> #1: the error message
>>
>> If current line is not fine,
>> 	printk(XENLOG_G_WARNING "Domain %d can't check reserved device
>> memory.\n", d->domain_id);
>>
>> I mean could you change this directly.
>
> This looks reasonable, albeit we generally prefer Dom%d or dom%d
> so that messages are somewhat grep-able.

Fixed.

>
>> #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?

Thanks
Tiejun