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Lu Baolu ea661ad6e1 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>
History 2022-05-13 15:14:56 +02:00
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Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.