iommu/vt-d: Size Page Request Queue to avoid overflow condition

PRQ overflow may cause I/O throughput congestion, resulting in unnecessary
degradation of I/O performance. Appropriately increasing the length of PRQ
can greatly reduce the occurrence of PRQ overflow. The count of maximum
page requests that can be generated in parallel by a PCIe device is
statically defined in the Outstanding Page Request Capacity field of the
PCIe ATS configure space.

The new length of PRQ is calculated by summing up the value of Outstanding
Page Request Capacity register across all devices where Page Requests are
supported on the real PR-capable platform (Intel Sapphire Rapids). The
result is round to the nearest higher power of 2.

The PRQ length is also double sized as the VT-d IOMMU driver only updates
the Page Request Queue Head Register (PQH_REG) after processing the entire
queue.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20220421113558.3504874-1-baolu.lu@linux.intel.com
Link: https://lore.kernel.org/r/20220510023407.2759143-5-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

include/linux/intel-svm.h

@@ -9,7 +9,7 @@
 #define __INTEL_SVM_H__
 
 /* Page Request Queue depth */
-#define PRQ_ORDER	2
+#define PRQ_ORDER	4
 #define PRQ_RING_MASK	((0x1000 << PRQ_ORDER) - 0x20)
 #define PRQ_DEPTH	((0x1000 << PRQ_ORDER) >> 5)