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

[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.stojac...@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