Re: [Xen-devel] [PATCH v12] introduce XENMEM_reserved_device_memory_map

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.07.15 at 14:55, <tiejun.chen@xxxxxxxxx> wrote:
>>  +#ifdef HAS_PASSTHROUGH
>> +        case XENMEM_reserved_device_memory_map:
>> +        {
>> +            struct get_reserved_device_memory grdm;
>> +
>> +            if ( unlikely(start_extent) )
>> +                return -ENOSYS;
>> +
>> +            if ( copy_from_guest(&grdm.map, compat, 1) ||
>> +                 !compat_handle_okay(grdm.map.buffer, grdm.map.nr_entries) 
> )
>> +                return -EFAULT;
>> +
>> +            if ( grdm.map.flags & ~XENMEM_RDM_ALL )
>> +                return -EINVAL;
>> +
>> +            grdm.used_entries = 0;
>> +            rc = 
>> iommu_get_reserved_device_memory(get_reserved_device_memory,
>> +                                                  &grdm);
>> +
> 
> Just as you asked me previously,
> 
> Here if RDM doesn't exist, so
> 
> grdm.map.nr_entries = grdm.used_entries = 0, and rc = 0, right?

No, grdm.map.nr_entries still holds whatever the caller passed.

>> +            if ( !rc && grdm.map.nr_entries < grdm.used_entries )
>> +                rc = -ENOBUFS;
>> +            grdm.map.nr_entries = grdm.used_entries;
>> +            if ( __copy_to_guest(compat, &grdm.map, 1) )
> 
> So can we still do this copy here?

We not only can, we need to. The only case where we might skip it
is when the incoming grdm.map.nr_entries is unchanged.

Jan


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