Re: [dpdk-dev] [PATCH v2] eal/bus: use RTE_IOVA_PA only if phys addresses are available

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 28-Oct-18 11:11 PM, Thomas Monjalon wrote:
> 17/09/2018 12:33, Burakov, Anatoly:
>> On 07-Sep-18 4:58 PM, Darek Stojaczyk wrote:
>>> When neither RTE_IOVA_VA nor RTE_IOVA_PA was explicitly
>>> requested, DPDK would currently fallback to the default
>>> RTE_IOVA_PA mode and possibly encounter a failure later
>>> on if running as a non-priviledged user. Attempting to
>>> use RTE_IOVA_VA if no phys addresses are available may
>>> help in this case.
>>>
>>> Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
>>> ---
>>> Changes since v1:
>>>    * added a missing rte_memory.h include
>>>
>>>    lib/librte_eal/common/eal_common_bus.c | 19 +++++++++++++++----
>>>    1 file changed, 15 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/lib/librte_eal/common/eal_common_bus.c b/lib/librte_eal/common/eal_common_bus.c
>>> index 0943851cc..68c581b8a 100644
>>> --- a/lib/librte_eal/common/eal_common_bus.c
>>> +++ b/lib/librte_eal/common/eal_common_bus.c
>>> @@ -37,6 +37,7 @@
>>>    #include <rte_bus.h>
>>>    #include <rte_debug.h>
>>>    #include <rte_string_fns.h>
>>> +#include <rte_memory.h>
>>>    
>>>    #include "eal_private.h"
>>>    
>>> @@ -236,9 +237,19 @@ rte_bus_get_iommu_class(void)
>>>    			mode |= bus->get_iommu_class();
>>>    	}
>>>    
>>> -	if (mode != RTE_IOVA_VA) {
>>> -		/* Use default IOVA mode */
>>> -		mode = RTE_IOVA_PA;
>>> +	if (mode == RTE_IOVA_VA)
>>> +		return RTE_IOVA_VA;
>>> +
>>> +	if (mode & RTE_IOVA_PA) {
>>> +		/* Not all buses support RTE_IOVA_VA, fallback to RTE_IOVA_PA */
>>> +		return RTE_IOVA_PA;
>>> +	}
>>> +
>>> +	if (rte_eal_using_phys_addrs()) {
>>> +		/* Default to RTE_IOVA_PA only if it's supported */
>>> +		return RTE_IOVA_PA;
>>>    	}
>>> -	return mode;
>>> +
>>> +	/* Since RTE_IOVA_PA is unsupported, fallback to RTE_IOVA_VA */
>>> +	return RTE_IOVA_VA;
>>>    }
>>>
>>
>> This is a good change, however I think that this is too pessimistic. If
>> i don't have any devices that explictly require IOVA_PA, i should be
>> running in IOVA_VA mode.
>>
>> This of course doesn't take hotplug into account, so a command-line
>> switch to force one or the other should also be available.
>>
>> For example, at startup, i might have devices bound to VFIO, so IOVA_VA
>> mode is picked. However, even though at a time of startup none of the
>> devices require physical addresses, i also know that i might later
>> hotplug a device that requires IOVA_PA (leaving the question of hotplug
>> brokenness aside for now...) - currently, this scenario will not work,
>> as i will be forced to use IOVA_VA mode unless i happen to have a
>> IOVA_PA device available at startup.
>>
>> Similarly, if i'm running DPDK as root but am only using virtual devices
>> like pcap, i should be able to force DPDK into using VA addresses [*],
>> yet currently i will be forced to use IOVA_PA if i don't *also* have a
>> few devices bound exclusively to VFIO.
>>
>> [*] Do we have vdev devices that require IOVA_PA? I can't think of any...
> 
> If running as root, what is the benefit of using virtual addresses?
> 

Contiguous memory addresses is one that comes to mind.

-- 
Thanks,
Anatoly