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

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

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...

-- 
Thanks,
Anatoly