[dpdk-dev] [PATCH v4 4/4] eal: select IOVA as VA mode for default case

[David Marchand]

From: Jerin Jacob <jerinj at marvell.com>

When bus layer reports the preferred mode as RTE_IOVA_DC then
select the RTE_IOVA_VA mode:

- All drivers work in RTE_IOVA_VA mode, irrespective of physical
address availability.

- By default, a mempool asks for IOVA-contiguous memory using
RTE_MEMZONE_IOVA_CONTIG. This is slow in RTE_IOVA_PA mode and it
may affect the application boot time.

Signed-off-by: Jerin Jacob <jerinj at marvell.com>
Acked-by: Anatoly Burakov <anatoly.burakov at intel.com>
Signed-off-by: David Marchand <david.marchand at redhat.com>
---
Changelog since v3:
- moved the explanations on RTE_IOVA_VA choice in the note section,
- reworded the comments on the PCI driver flag,

---
 doc/guides/prog_guide/env_abstraction_layer.rst | 22 ++++++++++++++++++++--
 lib/librte_eal/linux/eal/eal.c                  |  6 ++----
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/doc/guides/prog_guide/env_abstraction_layer.rst b/doc/guides/prog_guide/env_abstraction_layer.rst
index 1d63675..1487ea5 100644
--- a/doc/guides/prog_guide/env_abstraction_layer.rst
+++ b/doc/guides/prog_guide/env_abstraction_layer.rst
@@ -445,11 +445,29 @@ kernels.
 - if the preferred mode is RTE_IOVA_PA but there is no access to Physical
   Addresses, then EAL init fails early, since later probing of the devices
   would fail anyway,
-- if the preferred mode is RTE_IOVA_DC then based on the Physical Addresses
-  availability, the preferred mode is adjusted to RTE_IOVA_PA or RTE_IOVA_VA.
+- if the preferred mode is RTE_IOVA_DC then EAL selects the RTE_IOVA_VA mode.
   In the case when the buses had disagreed on the IOVA Mode at the first step,
   part of the buses won't work because of this decision.
 
+.. note::
+
+    The RTE_IOVA_VA mode is selected as the default for the following reasons:
+
+    - All drivers are expected to work in RTE_IOVA_VA mode, irrespective of
+      physical address availability.
+    - By default, the mempool, first asks for IOVA-contiguous memory using
+      ``RTE_MEMZONE_IOVA_CONTIG``. This is slow in RTE_IOVA_PA mode and it may
+      affect the application boot time.
+    - It is easy to enable large amount of IOVA-contiguous memory use-cases
+      with IOVA in VA mode.
+
+    It is expected that all PCI drivers work in both RTE_IOVA_PA and
+    RTE_IOVA_VA modes.
+
+    If a PCI driver does not support RTE_IOVA_PA mode, the
+    ``RTE_PCI_DRV_NEED_IOVA_AS_VA`` flag is used to dictate that this PCI
+    driver can only work in RTE_IOVA_VA mode.
+
 IOVA Mode Configuration
 ~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/lib/librte_eal/linux/eal/eal.c b/lib/librte_eal/linux/eal/eal.c
index 2e5499f..34db787 100644
--- a/lib/librte_eal/linux/eal/eal.c
+++ b/lib/librte_eal/linux/eal/eal.c
@@ -1061,10 +1061,8 @@ static void rte_eal_init_alert(const char *msg)
 		enum rte_iova_mode iova_mode = rte_bus_get_iommu_class();
 
 		if (iova_mode == RTE_IOVA_DC) {
-			iova_mode = phys_addrs ? RTE_IOVA_PA : RTE_IOVA_VA;
-			RTE_LOG(DEBUG, EAL,
-				"Buses did not request a specific IOVA mode, using '%s' based on physical addresses availability.\n",
-				phys_addrs ? "PA" : "VA");
+			iova_mode = RTE_IOVA_VA;
+			RTE_LOG(DEBUG, EAL, "Buses did not request a specific IOVA mode, select IOVA as VA mode.\n");
 		}
 #ifdef RTE_LIBRTE_KNI
 		/* Workaround for KNI which requires physical address to work */
-- 
1.8.3.1