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9388ccf699
Opt for devm_ioremap_wc() over devm_ioremap() when mapping the framebuffer.
Using devm_ioremap() results in the VA being mapped with PAT=UC-, which
considerably slows down drm_fb_memcpy(). In contrast, devm_ioremap_wc()
maps the VA with PAT set to WC, leading to better performance on platforms
where access to UC memory is much slower than WC memory.
Here's the performance data measured in a guest on the physical machine
"Sapphire Rapids XCC".
With host KVM honors guest PAT memory types, the effective memory type
for this framebuffer range is
- WC when devm_ioremap_wc() is used
- UC- when devm_ioremap() is used.
The data presented is an average from 10 execution runs.
Cycles: Avg cycles of executed bochs_primary_plane_helper_atomic_update()
from VM boot to GDM show up
Cnt: Avg cnt of executed bochs_primary_plane_helper_atomic_update()
from VM boot to GDM show up
T: Avg time of each bochs_primary_plane_helper_atomic_update().
-------------------------------------------------
| | devm_ioremap() | devm_ioremap_wc() |
|------------|----------------|-------------------|
| Cycles | 211.545M | 0.157M |
|------------|----------------|-------------------|
| Cnt | 142 | 1917 |
|------------|----------------|-------------------|
| T | 0.1748s | 0.0004s |
-------------------------------------------------
Note:
Following the rebase to [3], the previously reported GDM failure on the
VGA device [1] can no longer be reproduced, thanks to the memory management
improvements made in [2]. Despite this, I have proceeded to submit this
patch because of the noticeable performance improvements it provides.
Reported-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Closes: https://lore.kernel.org/all/87jzfutmfc.fsf@redhat.com/#t
Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://lore.kernel.org/all/87jzfutmfc.fsf@redhat.com/#t [1]
Link: https://patchwork.freedesktop.org/series/138086 [2]
Link: https://gitlab.freedesktop.org/drm/misc/kernel/-/tree/drm-misc-next [3]
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240909131643.28915-1-yan.y.zhao@intel.com
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
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https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText 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.